From: Ammar Faizi ammar.faizi@students.amikom.ac.id
Before this patch, the `_start` function looks like this: ``` 0000000000001170 <_start>: 1170: pop %rdi 1171: mov %rsp,%rsi 1174: lea 0x8(%rsi,%rdi,8),%rdx 1179: and $0xfffffffffffffff0,%rsp 117d: sub $0x8,%rsp 1181: call 1000 <main> 1186: movzbq %al,%rdi 118a: mov $0x3c,%rax 1191: syscall 1193: hlt 1194: data16 cs nopw 0x0(%rax,%rax,1) 119f: nop ``` Note the "and" to %rsp with $-16, it makes the %rsp be 16-byte aligned, but then there is a "sub" with $0x8 which makes the %rsp no longer 16-byte aligned, then it calls main. That's the bug!
What actually the x86-64 System V ABI mandates is that right before the "call", the %rsp must be 16-byte aligned, not after the "call". So the "sub" with $0x8 here breaks the alignment. Remove it.
An example where this rule matters is when the callee needs to align its stack at 16-byte for aligned move instruction, like `movdqa` and `movaps`. If the callee can't align its stack properly, it will result in segmentation fault.
x86-64 System V ABI also mandates the deepest stack frame should be zero. Just to be safe, let's zero the %rbp on startup as the content of %rbp may be unspecified when the program starts. Now it looks like this: ``` 0000000000001170 <_start>: 1170: pop %rdi 1171: mov %rsp,%rsi 1174: lea 0x8(%rsi,%rdi,8),%rdx 1179: xor %ebp,%ebp # zero the %rbp 117b: and $0xfffffffffffffff0,%rsp # align the %rsp 117f: call 1000 <main> 1184: movzbq %al,%rdi 1188: mov $0x3c,%rax 118f: syscall 1191: hlt 1192: data16 cs nopw 0x0(%rax,%rax,1) 119d: nopl (%rax) ```
Cc: Bedirhan KURT windowz414@gnuweeb.org Cc: Louvian Lyndal louvianlyndal@gmail.com Reported-by: Peter Cordes peter@cordes.ca Signed-off-by: Ammar Faizi ammar.faizi@students.amikom.ac.id [wt: I did this on purpose due to a misunderstanding of the spec, other archs will thus have to be rechecked, particularly i386] Cc: stable@vger.kernel.org Signed-off-by: Willy Tarreau w@1wt.eu --- tools/include/nolibc/nolibc.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h index 3430667b0d24..96b6d56acb57 100644 --- a/tools/include/nolibc/nolibc.h +++ b/tools/include/nolibc/nolibc.h @@ -399,14 +399,20 @@ struct stat { })
/* startup code */ +/* + * x86-64 System V ABI mandates: + * 1) %rsp must be 16-byte aligned right before the function call. + * 2) The deepest stack frame should be zero (the %rbp). + * + */ asm(".section .text\n" ".global _start\n" "_start:\n" "pop %rdi\n" // argc (first arg, %rdi) "mov %rsp, %rsi\n" // argv[] (second arg, %rsi) "lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx) - "and $-16, %rsp\n" // x86 ABI : esp must be 16-byte aligned when - "sub $8, %rsp\n" // entering the callee + "xor %ebp, %ebp\n" // zero the stack frame + "and $-16, %rsp\n" // x86 ABI : esp must be 16-byte aligned before call "call main\n" // main() returns the status code, we'll exit with it. "movzb %al, %rdi\n" // retrieve exit code from 8 lower bits "mov $60, %rax\n" // NR_exit == 60