6.18-stable review patch. If anyone has any objections, please let me know.
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From: Willy Tarreau w@1wt.eu
[ Upstream commit 2602949b22330f1275138e2b5aea5d49126b9757 ]
I recently got occasional build failures at -Os or -Oz that would always involve waitpid(), where the assembler would complain about this:
init.s: Error: .size expression for waitpid.constprop.0 does not evaluate to a constant
And without -fno-asynchronous-unwind-tables it could also spit such errors:
init.s:836: Error: CFI instruction used without previous .cfi_startproc init.s:838: Error: .cfi_endproc without corresponding .cfi_startproc init.s: Error: open CFI at the end of file; missing .cfi_endproc directive
A trimmed down reproducer is as simple as this:
int main(int argc, char **argv) { int ret, status;
if (argc == 0) ret = waitpid(-1, &status, 0); else ret = waitpid(-1, &status, 0);
return status; }
It produces the following asm code on x86_64:
.text .section .text.nolibc_memmove_memcpy .weak memmove .weak memcpy memmove: memcpy: movq %rdx, %rcx (...) retq .section .text.nolibc_memset .weak memset memset: xchgl %eax, %esi movq %rdx, %rcx pushq %rdi rep stosb popq %rax retq
.type waitpid.constprop.0.isra.0, @function waitpid.constprop.0.isra.0: subq $8, %rsp (...) jmp *.L5(,%rax,8) .section .rodata .align 8 .align 4 .L5: .quad .L10 (...) .quad .L4 .text .L10: (...) .cfi_def_cfa_offset 8 ret .cfi_endproc .LFE273: .size waitpid.constprop.0.isra.0, .-waitpid.constprop.0.isra.0
It's a bit dense, but here's the explanation: the compiler has emitted a ".text" statement because it knows it's working in the .text section.
Then, our hand-written asm code for the mem* functions forced the section to .text.something without the compiler knowing about it, so it thinks the code is still being emitted for .text. As such, without any .section statement, the waitpid.constprop.0.isra.0 label is in fact placed in the previously created section, here .text.nolibc_memset.
The waitpid() function involves a switch/case statement that can be turned to a jump table, which is what the compiler does with the .rodata section, and after that it restores .text, which is no longer the previous .text.nolibc_memset section. Then the CFI statements cross a section, so does the .size calculation, which explains the error.
While a first approach consisting in placing an explicit ".text" at the end of these functions was verified to work, it's still unreliable as it depends on what the compiler remembers having emitted previously. A better approach is to replace the ".section" with ".pushsection", and place a ".popsection" at the end, so that these code blocks are agnostic to where they're placed relative to other blocks.
Fixes: 553845eebd60 ("tools/nolibc: x86-64: Use `rep movsb` for `memcpy()` and `memmove()`") Fixes: 12108aa8c1a1 ("tools/nolibc: x86-64: Use `rep stosb` for `memset()`") Signed-off-by: Willy Tarreau w@1wt.eu Signed-off-by: Thomas Weißschuh linux@weissschuh.net Signed-off-by: Sasha Levin sashal@kernel.org --- tools/include/nolibc/arch-x86.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/tools/include/nolibc/arch-x86.h b/tools/include/nolibc/arch-x86.h index d3efc0c3b8adc..c8b0c3e624a51 100644 --- a/tools/include/nolibc/arch-x86.h +++ b/tools/include/nolibc/arch-x86.h @@ -351,7 +351,7 @@ void *memcpy(void *dst, const void *src, size_t len); void *memset(void *dst, int c, size_t len);
__asm__ ( -".section .text.nolibc_memmove_memcpy\n" +".pushsection .text.nolibc_memmove_memcpy\n" ".weak memmove\n" ".weak memcpy\n" "memmove:\n" @@ -371,8 +371,9 @@ __asm__ ( "rep movsb\n\t" "cld\n\t" "retq\n" +".popsection\n"
-".section .text.nolibc_memset\n" +".pushsection .text.nolibc_memset\n" ".weak memset\n" "memset:\n" "xchgl %eax, %esi\n\t" @@ -381,6 +382,7 @@ __asm__ ( "rep stosb\n\t" "popq %rax\n\t" "retq\n" +".popsection\n" );
#endif /* !defined(__x86_64__) */