Commit a08c5356 authored by Linus Torvalds's avatar Linus Torvalds

lib: add generic strnlen_user() function

This adds a new generic optimized strnlen_user() function that uses the
<asm/word-at-a-time.h> infrastructure to portably do efficient string
handling.

In many ways, strnlen is much simpler than strncpy, and in particular we
can always pre-align the words we load from memory.  That means that all
the worries about alignment etc are a non-issue, so this one can easily
be used on any architecture.  You obviously do have to do the
appropriate word-at-a-time.h macros.
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 36126f8f
...@@ -19,6 +19,9 @@ config RATIONAL ...@@ -19,6 +19,9 @@ config RATIONAL
config GENERIC_STRNCPY_FROM_USER config GENERIC_STRNCPY_FROM_USER
bool bool
config GENERIC_STRNLEN_USER
bool
config GENERIC_FIND_FIRST_BIT config GENERIC_FIND_FIRST_BIT
bool bool
......
...@@ -126,6 +126,7 @@ obj-$(CONFIG_CLZ_TAB) += clz_tab.o ...@@ -126,6 +126,7 @@ obj-$(CONFIG_CLZ_TAB) += clz_tab.o
obj-$(CONFIG_DDR) += jedec_ddr_data.o obj-$(CONFIG_DDR) += jedec_ddr_data.o
obj-$(CONFIG_GENERIC_STRNCPY_FROM_USER) += strncpy_from_user.o obj-$(CONFIG_GENERIC_STRNCPY_FROM_USER) += strncpy_from_user.o
obj-$(CONFIG_GENERIC_STRNLEN_USER) += strnlen_user.o
hostprogs-y := gen_crc32table hostprogs-y := gen_crc32table
clean-files := crc32table.h clean-files := crc32table.h
......
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/uaccess.h>
#include <asm/word-at-a-time.h>
/* Set bits in the first 'n' bytes when loaded from memory */
#ifdef __LITTLE_ENDIAN
# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
#else
# define aligned_byte_mask(n) (~0xfful << 8*(7-(n)))
#endif
/*
* Do a strnlen, return length of string *with* final '\0'.
* 'count' is the user-supplied count, while 'max' is the
* address space maximum.
*
* Return 0 for exceptions (which includes hitting the address
* space maximum), or 'count+1' if hitting the user-supplied
* maximum count.
*
* NOTE! We can sometimes overshoot the user-supplied maximum
* if it fits in a aligned 'long'. The caller needs to check
* the return value against "> max".
*/
static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
{
const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
long align, res = 0;
unsigned long c;
/*
* Truncate 'max' to the user-specified limit, so that
* we only have one limit we need to check in the loop
*/
if (max > count)
max = count;
/*
* Do everything aligned. But that means that we
* need to also expand the maximum..
*/
align = (sizeof(long) - 1) & (unsigned long)src;
src -= align;
max += align;
if (unlikely(__get_user(c,(unsigned long __user *)src)))
return 0;
c |= aligned_byte_mask(align);
for (;;) {
unsigned long data;
if (has_zero(c, &data, &constants)) {
data = prep_zero_mask(c, data, &constants);
data = create_zero_mask(data);
return res + find_zero(data) + 1 - align;
}
res += sizeof(unsigned long);
if (unlikely(max < sizeof(unsigned long)))
break;
max -= sizeof(unsigned long);
if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
return 0;
}
res -= align;
/*
* Uhhuh. We hit 'max'. But was that the user-specified maximum
* too? If so, return the marker for "too long".
*/
if (res >= count)
return count+1;
/*
* Nope: we hit the address space limit, and we still had more
* characters the caller would have wanted. That's 0.
*/
return 0;
}
/**
* strnlen_user: - Get the size of a user string INCLUDING final NUL.
* @str: The string to measure.
* @count: Maximum count (including NUL character)
*
* Context: User context only. This function may sleep.
*
* Get the size of a NUL-terminated string in user space.
*
* Returns the size of the string INCLUDING the terminating NUL.
* If the string is too long, returns 'count+1'.
* On exception (or invalid count), returns 0.
*/
long strnlen_user(const char __user *str, long count)
{
unsigned long max_addr, src_addr;
if (unlikely(count <= 0))
return 0;
max_addr = user_addr_max();
src_addr = (unsigned long)str;
if (likely(src_addr < max_addr)) {
unsigned long max = max_addr - src_addr;
return do_strnlen_user(str, count, max);
}
return 0;
}
EXPORT_SYMBOL(strnlen_user);
/**
* strlen_user: - Get the size of a user string INCLUDING final NUL.
* @str: The string to measure.
*
* Context: User context only. This function may sleep.
*
* Get the size of a NUL-terminated string in user space.
*
* Returns the size of the string INCLUDING the terminating NUL.
* On exception, returns 0.
*
* If there is a limit on the length of a valid string, you may wish to
* consider using strnlen_user() instead.
*/
long strlen_user(const char __user *str)
{
unsigned long max_addr, src_addr;
max_addr = user_addr_max();
src_addr = (unsigned long)str;
if (likely(src_addr < max_addr)) {
unsigned long max = max_addr - src_addr;
return do_strnlen_user(str, ~0ul, max);
}
return 0;
}
EXPORT_SYMBOL(strlen_user);
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment