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authorAmmar Faizi <ammar.faizi@students.amikom.ac.id>2021-10-24 19:28:14 +0200
committerPaul E. McKenney <paulmck@kernel.org>2021-11-30 17:26:42 -0800
commit937ed91c712273131de6d2a02caafd3ee84e0c72 (patch)
treea8dcbe468312f4cf2bee03b23c5344265c5b8e90 /tools/include
parent8c0abfd6d2f6b0221194241ac2908751a2a0385f (diff)
downloadlinux-937ed91c712273131de6d2a02caafd3ee84e0c72.tar.bz2
tools/nolibc: x86-64: Fix startup code bug
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> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'tools/include')
-rw-r--r--tools/include/nolibc/nolibc.h10
1 files 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