From 0749708352fddbe0fa81fc25f96e3b1f77c655f4 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 28 Apr 2012 14:27:38 -0700 Subject: x86: make word-at-a-time strncpy_from_user clear bytes at the end This makes the newly optimized x86 strncpy_from_user clear the final bytes in the word past the final NUL character, rather than copy them as the word they were in the source. NOTE! Unlike the silly semantics of the libc 'strncpy()' function, the kernel strncpy_from_user() has never cleared all of the end of the destination buffer. And neither does it do so now: it only clears the bytes at the end of the last word it copied. So why make this change at all? It doesn't really cost us anything extra (we have to calculate the mask to get the length anyway), and it means that *if* any user actually cares about zeroing the whole buffer, they can do a "memset()" before the strncpy_from_user(), and we will no longer write random bytes after the NUL character. In particular, the buffer contents will now at no point contain random source data from beyond the end of the string. In other words, it makes behavior a bit more repeatable at no new cost, so it's a small cleanup. I've been carrying this as a patch for the last few weeks or so in my tree (done at the same time the sign error was fixed in commit 12e993b89464), I might as well commit it. Signed-off-by: Linus Torvalds --- arch/x86/lib/usercopy.c | 20 ++++++++------------ 1 file changed, 8 insertions(+), 12 deletions(-) (limited to 'arch/x86') diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c index d6ae30bbd7b..2e4e4b02c37 100644 --- a/arch/x86/lib/usercopy.c +++ b/arch/x86/lib/usercopy.c @@ -44,13 +44,6 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n) } EXPORT_SYMBOL_GPL(copy_from_user_nmi); -static inline unsigned long count_bytes(unsigned long mask) -{ - mask = (mask - 1) & ~mask; - mask >>= 7; - return count_masked_bytes(mask); -} - /* * Do a strncpy, return length of string without final '\0'. * 'count' is the user-supplied count (return 'count' if we @@ -69,16 +62,19 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long max = count; while (max >= sizeof(unsigned long)) { - unsigned long c; + unsigned long c, mask; /* Fall back to byte-at-a-time if we get a page fault */ if (unlikely(__get_user(c,(unsigned long __user *)(src+res)))) break; - /* This can write a few bytes past the NUL character, but that's ok */ + mask = has_zero(c); + if (mask) { + mask = (mask - 1) & ~mask; + mask >>= 7; + *(unsigned long *)(dst+res) = c & mask; + return res + count_masked_bytes(mask); + } *(unsigned long *)(dst+res) = c; - c = has_zero(c); - if (c) - return res + count_bytes(c); res += sizeof(unsigned long); max -= sizeof(unsigned long); } -- cgit v1.2.3-70-g09d2