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![Tobin C. Harding](/assets/img/avatar_default.png)
printk specifier %p now hashes all addresses before printing. Sometimes we need to see the actual unmodified address. This can be achieved using %lx but then we face the risk that if in future we want to change the way the Kernel handles printing of pointers we will have to grep through the already existent 50 000 %lx call sites. Let's add specifier %px as a clear, opt-in, way to print a pointer and maintain some level of isolation from all the other hex integer output within the Kernel. Add printk specifier %px to print the actual unmodified address. Signed-off-by: Tobin C. Harding <me@tobin.cc>
483 lines
13 KiB
Text
483 lines
13 KiB
Text
=========================================
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How to get printk format specifiers right
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=========================================
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:Author: Randy Dunlap <rdunlap@infradead.org>
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:Author: Andrew Murray <amurray@mpc-data.co.uk>
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Integer types
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=============
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::
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If variable is of Type, use printk format specifier:
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------------------------------------------------------------
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int %d or %x
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unsigned int %u or %x
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long %ld or %lx
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unsigned long %lu or %lx
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long long %lld or %llx
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unsigned long long %llu or %llx
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size_t %zu or %zx
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ssize_t %zd or %zx
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s32 %d or %x
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u32 %u or %x
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s64 %lld or %llx
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u64 %llu or %llx
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If <type> is dependent on a config option for its size (e.g., ``sector_t``,
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``blkcnt_t``) or is architecture-dependent for its size (e.g., ``tcflag_t``),
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use a format specifier of its largest possible type and explicitly cast to it.
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Example::
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printk("test: sector number/total blocks: %llu/%llu\n",
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(unsigned long long)sector, (unsigned long long)blockcount);
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Reminder: ``sizeof()`` result is of type ``size_t``.
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The kernel's printf does not support ``%n``. For obvious reasons, floating
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point formats (``%e, %f, %g, %a``) are also not recognized. Use of any
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unsupported specifier or length qualifier results in a WARN and early
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return from vsnprintf.
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Raw pointer value SHOULD be printed with %p. The kernel supports
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the following extended format specifiers for pointer types:
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Pointer Types
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=============
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Pointers printed without a specifier extension (i.e unadorned %p) are
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hashed to give a unique identifier without leaking kernel addresses to user
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space. On 64 bit machines the first 32 bits are zeroed. If you _really_
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want the address see %px below.
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::
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%p abcdef12 or 00000000abcdef12
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Symbols/Function Pointers
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=========================
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::
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%pF versatile_init+0x0/0x110
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%pf versatile_init
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%pS versatile_init+0x0/0x110
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%pSR versatile_init+0x9/0x110
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(with __builtin_extract_return_addr() translation)
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%ps versatile_init
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%pB prev_fn_of_versatile_init+0x88/0x88
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The ``F`` and ``f`` specifiers are for printing function pointers,
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for example, f->func, &gettimeofday. They have the same result as
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``S`` and ``s`` specifiers. But they do an extra conversion on
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ia64, ppc64 and parisc64 architectures where the function pointers
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are actually function descriptors.
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The ``S`` and ``s`` specifiers can be used for printing symbols
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from direct addresses, for example, __builtin_return_address(0),
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(void *)regs->ip. They result in the symbol name with (``S``) or
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without (``s``) offsets. If KALLSYMS are disabled then the symbol
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address is printed instead.
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The ``B`` specifier results in the symbol name with offsets and should be
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used when printing stack backtraces. The specifier takes into
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consideration the effect of compiler optimisations which may occur
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when tail-call``s are used and marked with the noreturn GCC attribute.
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Examples::
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printk("Going to call: %pF\n", gettimeofday);
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printk("Going to call: %pF\n", p->func);
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printk("%s: called from %pS\n", __func__, (void *)_RET_IP_);
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printk("%s: called from %pS\n", __func__,
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(void *)__builtin_return_address(0));
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printk("Faulted at %pS\n", (void *)regs->ip);
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printk(" %s%pB\n", (reliable ? "" : "? "), (void *)*stack);
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Kernel Pointers
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===============
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::
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%pK 01234567 or 0123456789abcdef
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For printing kernel pointers which should be hidden from unprivileged
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users. The behaviour of ``%pK`` depends on the ``kptr_restrict sysctl`` - see
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Documentation/sysctl/kernel.txt for more details.
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Unmodified Addresses
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====================
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::
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%px 01234567 or 0123456789abcdef
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For printing pointers when you _really_ want to print the address. Please
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consider whether or not you are leaking sensitive information about the
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Kernel layout in memory before printing pointers with %px. %px is
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functionally equivalent to %lx. %px is preferred to %lx because it is more
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uniquely grep'able. If, in the future, we need to modify the way the Kernel
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handles printing pointers it will be nice to be able to find the call
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sites.
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Struct Resources
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================
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::
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%pr [mem 0x60000000-0x6fffffff flags 0x2200] or
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[mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
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%pR [mem 0x60000000-0x6fffffff pref] or
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[mem 0x0000000060000000-0x000000006fffffff pref]
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For printing struct resources. The ``R`` and ``r`` specifiers result in a
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printed resource with (``R``) or without (``r``) a decoded flags member.
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Passed by reference.
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Physical addresses types ``phys_addr_t``
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========================================
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::
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%pa[p] 0x01234567 or 0x0123456789abcdef
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For printing a ``phys_addr_t`` type (and its derivatives, such as
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``resource_size_t``) which can vary based on build options, regardless of
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the width of the CPU data path. Passed by reference.
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DMA addresses types ``dma_addr_t``
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==================================
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::
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%pad 0x01234567 or 0x0123456789abcdef
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For printing a ``dma_addr_t`` type which can vary based on build options,
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regardless of the width of the CPU data path. Passed by reference.
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Raw buffer as an escaped string
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===============================
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::
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%*pE[achnops]
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For printing raw buffer as an escaped string. For the following buffer::
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1b 62 20 5c 43 07 22 90 0d 5d
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few examples show how the conversion would be done (the result string
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without surrounding quotes)::
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%*pE "\eb \C\a"\220\r]"
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%*pEhp "\x1bb \C\x07"\x90\x0d]"
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%*pEa "\e\142\040\\\103\a\042\220\r\135"
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The conversion rules are applied according to an optional combination
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of flags (see :c:func:`string_escape_mem` kernel documentation for the
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details):
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- ``a`` - ESCAPE_ANY
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- ``c`` - ESCAPE_SPECIAL
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- ``h`` - ESCAPE_HEX
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- ``n`` - ESCAPE_NULL
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- ``o`` - ESCAPE_OCTAL
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- ``p`` - ESCAPE_NP
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- ``s`` - ESCAPE_SPACE
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By default ESCAPE_ANY_NP is used.
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ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
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printing SSIDs.
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If field width is omitted the 1 byte only will be escaped.
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Raw buffer as a hex string
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==========================
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::
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%*ph 00 01 02 ... 3f
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%*phC 00:01:02: ... :3f
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%*phD 00-01-02- ... -3f
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%*phN 000102 ... 3f
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For printing a small buffers (up to 64 bytes long) as a hex string with
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certain separator. For the larger buffers consider to use
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:c:func:`print_hex_dump`.
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MAC/FDDI addresses
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==================
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::
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%pM 00:01:02:03:04:05
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%pMR 05:04:03:02:01:00
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%pMF 00-01-02-03-04-05
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%pm 000102030405
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%pmR 050403020100
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For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
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specifiers result in a printed address with (``M``) or without (``m``) byte
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separators. The default byte separator is the colon (``:``).
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Where FDDI addresses are concerned the ``F`` specifier can be used after
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the ``M`` specifier to use dash (``-``) separators instead of the default
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separator.
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For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
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specifier to use reversed byte order suitable for visual interpretation
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of Bluetooth addresses which are in the little endian order.
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Passed by reference.
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IPv4 addresses
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==============
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::
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%pI4 1.2.3.4
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%pi4 001.002.003.004
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%p[Ii]4[hnbl]
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For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
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specifiers result in a printed address with (``i4``) or without (``I4``)
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leading zeros.
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The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
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host, network, big or little endian order addresses respectively. Where
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no specifier is provided the default network/big endian order is used.
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Passed by reference.
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IPv6 addresses
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==============
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::
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%pI6 0001:0002:0003:0004:0005:0006:0007:0008
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%pi6 00010002000300040005000600070008
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%pI6c 1:2:3:4:5:6:7:8
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For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
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specifiers result in a printed address with (``I6``) or without (``i6``)
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colon-separators. Leading zeros are always used.
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The additional ``c`` specifier can be used with the ``I`` specifier to
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print a compressed IPv6 address as described by
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http://tools.ietf.org/html/rfc5952
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Passed by reference.
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IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
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=========================================================
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::
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%pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
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%piS 001.002.003.004 or 00010002000300040005000600070008
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%pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
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%pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
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%p[Ii]S[pfschnbl]
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For printing an IP address without the need to distinguish whether it``s
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of type AF_INET or AF_INET6, a pointer to a valid ``struct sockaddr``,
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specified through ``IS`` or ``iS``, can be passed to this format specifier.
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The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
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(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
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flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
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In case of an IPv6 address the compressed IPv6 address as described by
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http://tools.ietf.org/html/rfc5952 is being used if the additional
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specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
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case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
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https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
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In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
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specifiers can be used as well and are ignored in case of an IPv6
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address.
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Passed by reference.
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Further examples::
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%pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
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%pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
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%pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
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UUID/GUID addresses
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===================
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::
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%pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
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%pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
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%pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
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%pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
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For printing 16-byte UUID/GUIDs addresses. The additional 'l', 'L',
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'b' and 'B' specifiers are used to specify a little endian order in
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lower ('l') or upper case ('L') hex characters - and big endian order
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in lower ('b') or upper case ('B') hex characters.
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Where no additional specifiers are used the default big endian
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order with lower case hex characters will be printed.
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Passed by reference.
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dentry names
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============
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::
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%pd{,2,3,4}
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%pD{,2,3,4}
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For printing dentry name; if we race with :c:func:`d_move`, the name might be
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a mix of old and new ones, but it won't oops. ``%pd`` dentry is a safer
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equivalent of ``%s`` ``dentry->d_name.name`` we used to use, ``%pd<n>`` prints
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``n`` last components. ``%pD`` does the same thing for struct file.
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Passed by reference.
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block_device names
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==================
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::
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%pg sda, sda1 or loop0p1
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For printing name of block_device pointers.
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struct va_format
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================
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::
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%pV
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For printing struct va_format structures. These contain a format string
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and va_list as follows::
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struct va_format {
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const char *fmt;
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va_list *va;
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};
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Implements a "recursive vsnprintf".
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Do not use this feature without some mechanism to verify the
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correctness of the format string and va_list arguments.
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Passed by reference.
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kobjects
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========
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::
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%pO
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Base specifier for kobject based structs. Must be followed with
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character for specific type of kobject as listed below:
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Device tree nodes:
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%pOF[fnpPcCF]
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For printing device tree nodes. The optional arguments are:
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f device node full_name
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n device node name
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p device node phandle
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P device node path spec (name + @unit)
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F device node flags
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c major compatible string
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C full compatible string
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Without any arguments prints full_name (same as %pOFf)
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The separator when using multiple arguments is ':'
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Examples:
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%pOF /foo/bar@0 - Node full name
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%pOFf /foo/bar@0 - Same as above
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%pOFfp /foo/bar@0:10 - Node full name + phandle
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%pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
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major compatible string +
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node flags
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D - dynamic
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d - detached
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P - Populated
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B - Populated bus
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Passed by reference.
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struct clk
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==========
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::
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%pC pll1
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%pCn pll1
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%pCr 1560000000
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For printing struct clk structures. ``%pC`` and ``%pCn`` print the name
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(Common Clock Framework) or address (legacy clock framework) of the
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structure; ``%pCr`` prints the current clock rate.
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Passed by reference.
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bitmap and its derivatives such as cpumask and nodemask
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=======================================================
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::
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%*pb 0779
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%*pbl 0,3-6,8-10
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For printing bitmap and its derivatives such as cpumask and nodemask,
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``%*pb`` output the bitmap with field width as the number of bits and ``%*pbl``
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output the bitmap as range list with field width as the number of bits.
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Passed by reference.
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Flags bitfields such as page flags, gfp_flags
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=============================================
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::
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%pGp referenced|uptodate|lru|active|private
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%pGg GFP_USER|GFP_DMA32|GFP_NOWARN
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%pGv read|exec|mayread|maywrite|mayexec|denywrite
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For printing flags bitfields as a collection of symbolic constants that
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would construct the value. The type of flags is given by the third
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character. Currently supported are [p]age flags, [v]ma_flags (both
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expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
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names and print order depends on the particular type.
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Note that this format should not be used directly in :c:func:`TP_printk()` part
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of a tracepoint. Instead, use the ``show_*_flags()`` functions from
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<trace/events/mmflags.h>.
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Passed by reference.
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Network device features
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=======================
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::
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%pNF 0x000000000000c000
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For printing netdev_features_t.
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Passed by reference.
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If you add other ``%p`` extensions, please extend lib/test_printf.c with
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one or more test cases, if at all feasible.
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Thank you for your cooperation and attention.
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