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Subject[PATCH 2/2] tools/nolibc: x86-64: Fix startup code bug
Date
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, 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!

Right before "call", the %rsp must be 16-byte aligned. So the "sub"
here breaks the alignment. Remove it.

Also the content of %rbp may be unspecified at process initialization
time. For example, if the _start gets called by an interpreter, the
interpreter may not zero the %rbp, so we should zero the %rbp on _start.

In this patch, we recode the _start function, and now it looks like
this:

0000000000001170 <_start>:
1170: pop %rdi # argc
1171: mov %rsp,%rsi # argv
1174: lea 0x8(%rsi,%rdi,8),%rdx # envp
1179: xor %ebp,%ebp # %rbp = 0
117b: and $0xfffffffffffffff0,%rsp # %rsp &= -16
117f: call 1000 <main>
1184: mov %eax,%edi
1186: mov $0xe7,%eax
118b: syscall # sys_exit_group(%rdi)
118d: hlt
118e: xchg %ax,%ax

Extra fixes:
- Use NR_exit_group instead of NR_exit.

- Use `mov %eax,%edi` instead of `movzbq %al,%rdi`. This makes the
exit code more observable from strace. While the exit code is
only 8-bit, the kernel has taken care of that, so no need to
worry about it.

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>
---
tools/include/nolibc/nolibc.h | 61 +++++++++++++++++++++++++++--------
1 file changed, 47 insertions(+), 14 deletions(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 1483d95c8330..f502b645d363 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -403,21 +403,54 @@ struct stat {
_ret; \
})

+
/* startup code */
-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
- "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
- "syscall\n" // really exit
- "hlt\n" // ensure it does not return
- "");
+asm(
+ ".section .text\n"
+ ".global _start\n"
+
+ "_start:\n\t"
+ "popq %rdi\n\t" // argc (first arg, %rdi)
+ "movq %rsp, %rsi\n\t" // argv[] (second arg, %rsi)
+ "leaq 8(%rsi,%rdi,8), %rdx\n\t" // then a NULL, then envp (third arg, %rdx)
+
+ /*
+ * The System V ABI mandates the deepest stack frame should be zero.
+ * Thus we zero the %rbp here.
+ */
+ "xorl %ebp, %ebp\n\t"
+
+ /*
+ * The System V ABI mandates the %rsp needs to be aligned at 16-byte
+ * before performing a function call.
+ */
+ "andq $-16, %rsp\n\t"
+
+ /*
+ * main() returns the status code, we will exit with it.
+ */
+ "callq main\n\t"
+
+ /*
+ * Move the return value to the first argument of exit_group.
+ */
+ "movl %eax, %edi\n\t"
+
+ /*
+ * NR_exit_group == 231
+ */
+ "movl $231, %eax\n\t"
+
+ /*
+ * Really exit.
+ */
+ "syscall\n\t"
+
+ /*
+ * Ensure it does not return.
+ */
+ "hlt\n\t"
+);

/* fcntl / open */
#define O_RDONLY 0
--
2.30.2
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