[PATCH 3/3] tools/nolibc: fix incorrect truncation of exit code

Willy Tarreau
2021-10-24T17:29:22Z

Ammar Faizi reported that our exit code handling is wrong. We truncate
it to the lowest 8 bits but the syscall itself is expected to take a
regular 32-bit signed integer, not an unsigned char. It's the kernel
that later truncates it to the lowest 8 bits. The difference is visible
in strace, where the program below used to show exit(255) instead of
exit(-1):

int main(void)
{
return -1;
}

This patch applies the fix to all archs. x86_64, i386, arm64, armv7 and
mips were all tested and confirmed to work fine now. Risc-v was not
tested but the change is trivial and exactly the same as for other archs.

Reported-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
Cc: stable@vger.kernel.org
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
tools/include/nolibc/nolibc.h | 13 +++++--------
1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 7f300dc379e7..3e2c6f2ed587 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -414,7 +414,7 @@ asm(".section .text\n"
"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 %eax, %edi\n" // retrieve exit code (32 bit)
"mov $60, %rax\n" // NR_exit == 60
"syscall\n" // really exit
"hlt\n" // ensure it does not return
@@ -602,9 +602,9 @@ asm(".section .text\n"
"push %ebx\n" // support both regparm and plain stack modes
"push %eax\n"
"call main\n" // main() returns the status code in %eax
- "movzbl %al, %ebx\n" // retrieve exit code from lower 8 bits
- "movl $1, %eax\n" // NR_exit == 1
- "int $0x80\n" // exit now
+ "mov %eax, %ebx\n" // retrieve exit code (32-bit int)
+ "movl $1, %eax\n" // NR_exit == 1
+ "int $0x80\n" // exit now
"hlt\n" // ensure it does not
"");

@@ -788,7 +788,6 @@ asm(".section .text\n"
"and %r3, %r1, $-8\n" // AAPCS : sp must be 8-byte aligned in the
"mov %sp, %r3\n" // callee, an bl doesn't push (lr=pc)
"bl main\n" // main() returns the status code, we'll exit with it.
- "and %r0, %r0, $0xff\n" // limit exit code to 8 bits
"movs r7, $1\n" // NR_exit == 1
"svc $0x00\n"
"");
@@ -985,7 +984,6 @@ asm(".section .text\n"
"add x2, x2, x1\n" // + argv
"and sp, x1, -16\n" // sp must be 16-byte aligned in the callee
"bl main\n" // main() returns the status code, we'll exit with it.
- "and x0, x0, 0xff\n" // limit exit code to 8 bits
"mov x8, 93\n" // NR_exit == 93
"svc #0\n"
"");
@@ -1190,7 +1188,7 @@ asm(".section .text\n"
"addiu $sp,$sp,-16\n" // the callee expects to save a0..a3 there!
"jal main\n" // main() returns the status code, we'll exit with it.
"nop\n" // delayed slot
- "and $a0, $v0, 0xff\n" // limit exit code to 8 bits
+ "move $a0, $v0\n" // retrieve 32-bit exit code from v0
"li $v0, 4001\n" // NR_exit == 4001
"syscall\n"
".end __start\n"
@@ -1388,7 +1386,6 @@ asm(".section .text\n"
"add a2,a2,a1\n" // + argv
"andi sp,a1,-16\n" // sp must be 16-byte aligned
"call main\n" // main() returns the status code, we'll exit with it.
- "andi a0, a0, 0xff\n" // limit exit code to 8 bits
"li a7, 93\n" // NR_exit == 93
"ecall\n"
"");
--
2.17.5