[PATCH v2 16/22] tools/nolibc: add auxiliary vector retrieval for arm

Willy Tarreau
2023-01-10T07:25:14Z

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>

It was tested in arm, thumb1 and thumb2 modes.
---
tools/include/nolibc/arch-arm.h | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/tools/include/nolibc/arch-arm.h b/tools/include/nolibc/arch-arm.h
index 79666b590e87..42499f23e73c 100644
--- a/tools/include/nolibc/arch-arm.h
+++ b/tools/include/nolibc/arch-arm.h
@@ -197,6 +197,7 @@ struct sys_stat_struct {
})

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

/* startup code */
void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
@@ -211,6 +212,16 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
"ldr %r3, 1f\n" // r3 = &environ (see below)
"str %r2, [r3]\n" // store envp into environ

+ "mov r4, r2\n" // search for auxv (follows NULL after last env)
+ "0:\n"
+ "mov r5, r4\n" // r5 = r4
+ "add r4, r4, #4\n" // r4 += 4
+ "ldr r5,[r5]\n" // r5 = *r5 = *(r4-4)
+ "cmp r5, #0\n" // and stop at NULL after last env
+ "bne 0b\n"
+ "ldr %r3, 2f\n" // r3 = &_auxv (low bits)
+ "str r4, [r3]\n" // store r4 into _auxv
+
"mov %r3, $8\n" // AAPCS : sp must be 8-byte aligned in the
"neg %r3, %r3\n" // callee, and bl doesn't push (lr=pc)
"and %r3, %r3, %r1\n" // so we do sp = r1(=sp) & r3(=-8);
@@ -222,6 +233,8 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
".align 2\n" // below are the pointers to a few variables
"1:\n"
".word environ\n"
+ "2:\n"
+ ".word _auxv\n"
);
__builtin_unreachable();
}
--
2.17.5