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author | Linus Torvalds <torvalds@linux-foundation.org> | 2012-05-26 20:06:38 +0200 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2012-05-26 20:33:53 +0200 |
commit | a08c5356a3aaf638c41897ae4169de18db89595e (patch) | |
tree | fe0d1cb48a26cc000c199d484a139799d559a178 /lib/strnlen_user.c | |
parent | word-at-a-time: make the interfaces truly generic (diff) | |
download | linux-a08c5356a3aaf638c41897ae4169de18db89595e.tar.xz linux-a08c5356a3aaf638c41897ae4169de18db89595e.zip |
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: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib/strnlen_user.c')
-rw-r--r-- | lib/strnlen_user.c | 138 |
1 files changed, 138 insertions, 0 deletions
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c new file mode 100644 index 000000000000..90900ecfeb54 --- /dev/null +++ b/lib/strnlen_user.c @@ -0,0 +1,138 @@ +#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); |