[PATCH 1/3] tools/nolibc: x86-64: Fix startup code bug

Willy Tarreau
2021-10-24T17:29:23Z

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
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
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
--
2.17.5