[PATCH v2 14/22] tools/nolibc: add auxiliary vector retrieval for x86_64

Willy Tarreau
2023-01-10T07:30:05Z

In the _start block we now iterate over envp to find the auxiliary
vector after the NULL. The pointer is saved into an _auxv variable
that is marked as weak so that it's accessible from multiple units.

Signed-off-by: Willy Tarreau <w@1wt.eu>
---
tools/include/nolibc/arch-x86_64.h | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h
index 683702a16a61..17f6751208e7 100644
--- a/tools/include/nolibc/arch-x86_64.h
+++ b/tools/include/nolibc/arch-x86_64.h
@@ -179,6 +179,7 @@ struct sys_stat_struct {
})

char **environ __attribute__((weak));
+const unsigned long *_auxv __attribute__((weak));

/* startup code */
/*
@@ -195,6 +196,12 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
"lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx)
"mov %rdx, environ\n" // save environ
"xor %ebp, %ebp\n" // zero the stack frame
+ "mov %rdx, %rax\n" // search for auxv (follows NULL after last env)
+ "0:\n"
+ "add $8, %rax\n" // search for auxv using rax, it follows the
+ "cmp -8(%rax), %rbp\n" // ... NULL after last env (rbp is zero here)
+ "jnz 0b\n"
+ "mov %rax, _auxv\n" // save it into _auxv
"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.
"mov %eax, %edi\n" // retrieve exit code (32 bit)
--
2.17.5