linux-kernel - [PATCH v2 02/15] tools/nolibc: mips: load current function to $t9

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Message-Id: <20240807-nolibc-llvm-v2-2-c20f2f5fc7c2@weissschuh.net>
Date: Wed, 07 Aug 2024 23:51:38 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Willy Tarreau <w@....eu>, Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH v2 02/15] tools/nolibc: mips: load current function to $t9

The MIPS calling convention requires the address of the current function
to be available in $t9.
This was not done so far.

For GCC this seems to have worked, but when compiled with clang the
executable segfault instantly.
Properly load the address of _start_c() into $t9 before calling it.

Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
---
 tools/include/nolibc/arch-mips.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
index 62cc50ef3288..a2ee77ed2fbb 100644
--- a/tools/include/nolibc/arch-mips.h
+++ b/tools/include/nolibc/arch-mips.h
@@ -194,7 +194,9 @@ void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_
 		"li    $t0, -8\n"
 		"and   $sp, $sp, $t0\n"  /* $sp must be 8-byte aligned                     */
 		"addiu $sp, $sp, -16\n"  /* the callee expects to save a0..a3 there        */
-		"jal   _start_c\n"       /* transfer to c runtime                          */
+		"lui $t9, %hi(_start_c)\n" /* ABI requires current function address in $t9 */
+		"ori $t9, %lo(_start_c)\n"
+		"jalr $t9\n"             /* transfer to c runtime                          */
 		" nop\n"                 /* delayed slot                                   */
 		".set pop\n"
 	);

-- 
2.46.0