On Tue, Aug 18, 2020 at 09:10:01AM +0200, Ard Biesheuvel wrote:
On Tue, 18 Aug 2020 at 00:02, Nick Desaulniers ndesaulniers@google.com 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. This optimization was introduced into clang-12. Because the kernel does not provide an implementation of stpcpy, we observe linkage failures for almost all targets when building with ToT clang.
The interface is unsafe as it does not perform any bounds checking. Disable this "libcall optimization" via `-fno-builtin-stpcpy`.
Unlike commit 5f074f3e192f ("lib/string.c: implement a basic bcmp") which cited failures with `-fno-builtin-*` flags being retained in LLVM LTO, that bug seems to have been fixed by https://reviews.llvm.org/D71193, so the above sha can now be reverted in favor of `-fno-builtin-bcmp`.
Cc: stable@vger.kernel.org # 4.4
Why does a fix for Clang-12 have to be backported all the way to v4.4? How does that meet the requirements for stable patches?
Because people like to build older kernels with new compliler versions.
And those "people" include me, who doesn't want to keep around old compilers just because my distro moved to the latest one...
We've been doing this for the past 4+ years, for new versions of gcc, keeping 4.4.y building properly with the bleeding edge of that compiler, why is clang any different here?
thanks,
greg k-h