On Sat, Aug 15, 2020 at 2:24 PM Joe Perches joe@perches.com wrote:
On Sat, 2020-08-15 at 13:47 -0700, Nick Desaulniers wrote:
On Sat, Aug 15, 2020 at 9:34 AM Kees Cook keescook@chromium.org wrote:
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.
O_O What?
No; this is a _terrible_ API: there is no bounds checking, there are no buffer sizes. Anything using the example sprintf() pattern is _already_ wrong and must be removed from the kernel. (Yes, I realize that the kernel is *filled* with this bad assumption that "I'll never write more than PAGE_SIZE bytes to this buffer", but that's both theoretically wrong ("640k is enough for anybody") and has been known to be wrong in practice too (e.g. when suddenly your writing routine is reachable by splice(2) and you may not have a PAGE_SIZE buffer).
But we cannot _add_ another dangerous string API. We're already in a terrible mess trying to remove strcpy[1], strlcpy[2], and strncpy[3]. This needs to be addressed up by removing the unbounded sprintf() uses. (And to do so without introducing bugs related to using snprintf() when scnprintf() is expected[4].)
Well, everything (-next, mainline, stable) is broken right now (with ToT Clang) without providing this symbol. I'm not going to go clean the entire kernel's use of sprintf to get our CI back to being green.
Maybe this should get place in compiler-clang.h so it isn't generic and public.
https://bugs.llvm.org/show_bug.cgi?id=47162#c7 and https://bugs.llvm.org/show_bug.cgi?id=47144 Seem to imply that Clang is not the only compiler that can lower a sequence of libcalls to stpcpy. Do we want to wait until we have a fire drill w/ GCC to move such an implementation from include/linux/compiler-clang.h back in to lib/string.c?
Something like:
include/linux/compiler-clang.h | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+)
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h index cee0c728d39a..6279f1904e39 100644 --- a/include/linux/compiler-clang.h +++ b/include/linux/compiler-clang.h @@ -61,3 +61,30 @@ #if __has_feature(shadow_call_stack) # define __noscs __attribute__((__no_sanitize__("shadow-call-stack"))) #endif
+#ifndef __HAVE_ARCH_STPCPY +/**
- stpcpy - copy a string from src to dest returning a pointer to the new end
of dest, including src's NULL terminator. May overrun dest.
- @dest: pointer to buffer being copied into.
Must be large enough to receive copy.
- @src: pointer to the beginning of string being copied from.
Must not overlap dest.
- This function exists _only_ to support clang's possible conversion of
- sprintf calls to stpcpy.
- stpcpy differs from strcpy in two key ways:
- inputs must not overlap.
- return value is dest's NUL termination character after copy.
- (for strcpy, the return value is a pointer to src)
- */
+static inline char *stpcpy(char __restrict *dest, const char __restrict *src) +{
while ((*dest++ = *src++) != '\0') {; /* nothing */}return --dest;+} +#endif