On Fri, Aug 14, 2020 at 07:09:44PM -0700, Nick Desaulniers wrote:
LLVM implemented a recent "libcall optimization" that lowers calls to `sprintf(dest, "%s", str)` where the return value is used to `stpcpy(dest, str) - dest`. This generally avoids the machinery involved in parsing format strings. Calling `sprintf` with overlapping arguments was clarified in ISO C99 and POSIX.1-2001 to be undefined behavior.
`stpcpy` is just like `strcpy` except it returns the pointer to the new tail of `dest`. This allows you to chain multiple calls to `stpcpy` in one statement.
`stpcpy` was first standardized in POSIX.1-2008.
Implement this so that we don't observe linkage failures due to missing symbol definitions for `stpcpy`.
Similar to last year's fire drill with: commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
This optimization was introduced into clang-12.
Cc: stable@vger.kernel.org Link: https://bugs.llvm.org/show_bug.cgi?id=47162 Link: https://github.com/ClangBuiltLinux/linux/issues/1126 Link: https://man7.org/linux/man-pages/man3/stpcpy.3.html Link: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stpcpy.html Link: https://reviews.llvm.org/D85963 Suggested-by: Arvind Sankar nivedita@alum.mit.edu Suggested-by: Joe Perches joe@perches.com Reported-by: Sami Tolvanen samitolvanen@google.com Tested-by: Sami Tolvanen samitolvanen@google.com Signed-off-by: Nick Desaulniers ndesaulniers@google.com
Changes V2:
- Added Sami's Tested by; though the patch changed implementation, the missing symbol at link time was the problem Sami was observing.
- Fix __restrict -> __restrict__ typo as per Joe.
- Drop note about restrict from commit message as per Arvind.
- Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
- Fix off by one error as per Arvind; I had another off by one error in my test program that was masking this.
include/linux/string.h | 3 +++ lib/string.c | 23 +++++++++++++++++++++++ 2 files changed, 26 insertions(+)
diff --git a/include/linux/string.h b/include/linux/string.h index b1f3894a0a3e..7686dbca8582 100644 --- a/include/linux/string.h +++ b/include/linux/string.h @@ -31,6 +31,9 @@ size_t strlcpy(char *, const char *, size_t); #ifndef __HAVE_ARCH_STRSCPY ssize_t strscpy(char *, const char *, size_t); #endif +#ifndef __HAVE_ARCH_STPCPY +extern char *stpcpy(char *__restrict__, const char *__restrict__); +#endif /* Wraps calls to strscpy()/memset(), no arch specific code required */ ssize_t strscpy_pad(char *dest, const char *src, size_t count); diff --git a/lib/string.c b/lib/string.c index 6012c385fb31..68ddbffbbd58 100644 --- a/lib/string.c +++ b/lib/string.c @@ -272,6 +272,29 @@ ssize_t strscpy_pad(char *dest, const char *src, size_t count) } EXPORT_SYMBOL(strscpy_pad); +#ifndef __HAVE_ARCH_STPCPY +/**
- stpcpy - copy a string from src to dest returning a pointer to the new end
of dest, including src's NUL terminator. May overrun dest.
- @dest: pointer to end of string being copied into. Must be large enough
to receive copy.
- @src: pointer to the beginning of string being copied from. Must not overlap
dest.
- stpcpy differs from strcpy in two key ways:
- inputs must not overlap.
Looks like you missed my second email: strcpy also does not allow inputs to overlap. Couple typos below.
- return value is the new NULL terminated character. (for strcpy, the
^^ NUL terminator.
- return value is a pointer to src.
^^ dest.)
- */
+#undef stpcpy +char *stpcpy(char *__restrict__ dest, const char *__restrict__ src) +{
- while ((*dest++ = *src++) != '\0')
/* nothing */;- return --dest;
+} +#endif
#ifndef __HAVE_ARCH_STRCAT /**
- strcat - Append one %NUL-terminated string to another
-- 2.28.0.220.ged08abb693-goog
The kernel-doc comments in string.c currently have a mix of %NUL and NUL, but the former seems to be more common. %NUL-terminator appears to be the preferred wording.