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On 64bit architectures that support 32bit processes there are two possible layouts for NT_PRSTATUS note in ELF coredumps. For one thing, several fields are 64bit for native processes and 32bit for compat ones (pr_sigpend, etc.). For another, the register dump is obviously different - the size and number of registers are not going to be the same for 32bit and 64bit variants of processor. Usually that's handled by having two structures - elf_prstatus for native layout and compat_elf_prstatus for 32bit one. 32bit processes are handled by fs/compat_binfmt_elf.c, which defines a macro called 'elf_prstatus' that expands to compat_elf_prstatus. Then it includes fs/binfmt_elf.c, which makes all references to struct elf_prstatus to be textually replaced with struct compat_elf_prstatus. Ugly and somewhat brittle, but it works. However, amd64 is worse - there are _three_ possible layouts. One for native 64bit processes, another for i386 (32bit) processes and yet another for x32 (32bit address space with full 64bit registers). Both i386 and x32 processes are handled by fs/compat_binfmt_elf.c, with usual compat_binfmt_elf.c trickery. However, the layouts for i386 and x32 are not identical - they have the common beginning, but the register dump part (pr_reg) is bigger on x32. Worse, pr_reg is not the last field - it's followed by int pr_fpvalid, so that field ends up at different offsets for i386 and x32 layouts. Fortunately, there's not much code that cares about any of that - it's all encapsulated in fill_thread_core_info(). Since x32 variant is bigger, we define compat_elf_prstatus to match that layout. That way i386 processes have enough space to fit their layout into. Moreover, since these layouts are identical prior to pr_reg, we don't need to distinguish x32 and i386 cases when we are setting the fields prior to pr_reg. Filling pr_reg itself is done by calling ->get() method of appropriate regset, and that method knows what layout (and size) to use. We do need to distinguish x32 and i386 cases only for two things: setting ->pr_fpvalid (offset differs for x32 and i386) and choosing the right size for our note. The way it's done is Not Nice, for the lack of more accurate printable description. There are two macros (PRSTATUS_SIZE and SET_PR_FPVALID), that default essentially to sizeof(struct elf_prstatus) and (S)->pr_fpvalid = 1. On x86 asm/compat.h provides its own variants. Unfortunately, quite a few things go wrong there: * PRSTATUS_SIZE doesn't use the normal test for process being an x32 one; it compares the size reported by regset with the size of pr_reg. * it hardcodes the sizes of x32 and i386 variants (296 and 144 resp.), so if some change in includes leads to asm/compat.h pulled in by fs/binfmt_elf.c we are in trouble - it will end up using the size of x32 variant for 64bit processes. * it's in the wrong place; asm/compat.h couldn't define the structure for i386 layout, since it lacks quite a few types needed for it. Hardcoded sizes are largely due to that. The proper fix would be to have an explicitly defined i386 variant of structure and have PRSTATUS_SIZE/SET_PR_FPVALID check for TIF_X32 to choose the variant that should be used. Unfortunately, that requires some manipulations of headers; we'll do that later in the series, but for now let's go with the minimal variant - rename PRSTATUS_SIZE in asm/compat.h to COMPAT_PRSTATUS_SIZE, have fs/compat_binfmt_elf.c define PRSTATUS_SIZE to COMPAT_PRSTATUS_SIZE and use the normal TIF_X32 check in that macro. The size of i386 variant is kept hardcoded for now. Similar story for SET_PR_FPVALID. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.