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![Linus Torvalds](/assets/img/avatar_default.png)
This converts the generic user string functions to use the batched user access functions. It makes a big difference on Skylake, which is the first x86 microarchitecture to implement SMAP. The STAC/CLAC instructions are not very fast, and doing them for each access inside the loop that copies strings from user space (which is what the pathname handling does for every pathname the kernel uses, for example) is very inefficient. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
158 lines
4.3 KiB
C
158 lines
4.3 KiB
C
#include <linux/kernel.h>
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#include <linux/export.h>
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#include <linux/uaccess.h>
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#include <asm/word-at-a-time.h>
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/* Set bits in the first 'n' bytes when loaded from memory */
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#ifdef __LITTLE_ENDIAN
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# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
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#else
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# define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
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#endif
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/*
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* Do a strnlen, return length of string *with* final '\0'.
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* 'count' is the user-supplied count, while 'max' is the
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* address space maximum.
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*
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* Return 0 for exceptions (which includes hitting the address
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* space maximum), or 'count+1' if hitting the user-supplied
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* maximum count.
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*
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* NOTE! We can sometimes overshoot the user-supplied maximum
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* if it fits in a aligned 'long'. The caller needs to check
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* the return value against "> max".
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*/
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static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
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{
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const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
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long align, res = 0;
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unsigned long c;
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/*
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* Truncate 'max' to the user-specified limit, so that
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* we only have one limit we need to check in the loop
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*/
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if (max > count)
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max = count;
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/*
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* Do everything aligned. But that means that we
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* need to also expand the maximum..
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*/
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align = (sizeof(long) - 1) & (unsigned long)src;
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src -= align;
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max += align;
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if (unlikely(unsafe_get_user(c,(unsigned long __user *)src)))
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return 0;
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c |= aligned_byte_mask(align);
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for (;;) {
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unsigned long data;
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if (has_zero(c, &data, &constants)) {
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data = prep_zero_mask(c, data, &constants);
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data = create_zero_mask(data);
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return res + find_zero(data) + 1 - align;
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}
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res += sizeof(unsigned long);
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/* We already handled 'unsigned long' bytes. Did we do it all ? */
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if (unlikely(max <= sizeof(unsigned long)))
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break;
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max -= sizeof(unsigned long);
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if (unlikely(unsafe_get_user(c,(unsigned long __user *)(src+res))))
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return 0;
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}
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res -= align;
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/*
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* Uhhuh. We hit 'max'. But was that the user-specified maximum
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* too? If so, return the marker for "too long".
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*/
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if (res >= count)
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return count+1;
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/*
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* Nope: we hit the address space limit, and we still had more
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* characters the caller would have wanted. That's 0.
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*/
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return 0;
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}
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/**
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* strnlen_user: - Get the size of a user string INCLUDING final NUL.
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* @str: The string to measure.
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* @count: Maximum count (including NUL character)
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*
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* Context: User context only. This function may sleep if pagefaults are
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* enabled.
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*
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* Get the size of a NUL-terminated string in user space.
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*
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* Returns the size of the string INCLUDING the terminating NUL.
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* If the string is too long, returns a number larger than @count. User
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* has to check the return value against "> count".
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* On exception (or invalid count), returns 0.
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*
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* NOTE! You should basically never use this function. There is
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* almost never any valid case for using the length of a user space
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* string, since the string can be changed at any time by other
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* threads. Use "strncpy_from_user()" instead to get a stable copy
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* of the string.
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*/
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long strnlen_user(const char __user *str, long count)
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{
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unsigned long max_addr, src_addr;
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if (unlikely(count <= 0))
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return 0;
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max_addr = user_addr_max();
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src_addr = (unsigned long)str;
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if (likely(src_addr < max_addr)) {
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unsigned long max = max_addr - src_addr;
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long retval;
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user_access_begin();
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retval = do_strnlen_user(str, count, max);
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user_access_end();
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return retval;
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}
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return 0;
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}
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EXPORT_SYMBOL(strnlen_user);
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/**
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* strlen_user: - Get the size of a user string INCLUDING final NUL.
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* @str: The string to measure.
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*
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* Context: User context only. This function may sleep if pagefaults are
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* enabled.
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*
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* Get the size of a NUL-terminated string in user space.
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*
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* Returns the size of the string INCLUDING the terminating NUL.
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* On exception, returns 0.
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*
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* If there is a limit on the length of a valid string, you may wish to
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* consider using strnlen_user() instead.
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*/
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long strlen_user(const char __user *str)
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{
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unsigned long max_addr, src_addr;
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max_addr = user_addr_max();
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src_addr = (unsigned long)str;
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if (likely(src_addr < max_addr)) {
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unsigned long max = max_addr - src_addr;
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long retval;
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user_access_begin();
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retval = do_strnlen_user(str, ~0ul, max);
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user_access_end();
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return retval;
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}
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return 0;
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}
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EXPORT_SYMBOL(strlen_user);
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