linux-kernel - [PATCH v2] tools/nolibc: mips: add support for PIC

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Date:   Wed, 08 Nov 2023 19:14:43 +0100
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] tools/nolibc: mips: add support for PIC

MIPS requires some extra instructions to set up the $gp register for the
with a pointer to the global data area.

This isn't needed for non-PIC builds, but this patch enables the code
unconditionally to prevent bitrot.

Also enable PIC in one of the test configurations for ongoing
validation.

Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
---
Changes in v2:
- Preserver 8-byte alignment of stack pointer
- Link to v1: https://lore.kernel.org/r/20231108-nolibc-pic-v1-1-9b7a429d5a6d@weissschuh.net
---
 tools/include/nolibc/arch-mips.h        | 7 ++++++-
 tools/testing/selftests/nolibc/Makefile | 2 +-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
index 3a2c76716b83..62cc50ef3288 100644
--- a/tools/include/nolibc/arch-mips.h
+++ b/tools/include/nolibc/arch-mips.h
@@ -184,8 +184,13 @@ void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_
 	__asm__ volatile (
 		".set push\n"
 		".set noreorder\n"
-		".option pic0\n"
+		"bal 1f\n"               /* prime $ra for .cpload                            */
+		"nop\n"
+		"1:\n"
+		".cpload $ra\n"
 		"move  $a0, $sp\n"       /* save stack pointer to $a0, as arg1 of _start_c */
+		"addiu $sp, $sp, -4\n"   /* space for .cprestore to store $gp              */
+		".cprestore 0\n"
 		"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        */
diff --git a/tools/testing/selftests/nolibc/Makefile b/tools/testing/selftests/nolibc/Makefile
index 508435b8ac2a..484bb02d8e6c 100644
--- a/tools/testing/selftests/nolibc/Makefile
+++ b/tools/testing/selftests/nolibc/Makefile
@@ -144,7 +144,7 @@ CFLAGS_ppc = -m32 -mbig-endian -mno-vsx $(call cc-option,-mmultiple)
 CFLAGS_ppc64 = -m64 -mbig-endian -mno-vsx $(call cc-option,-mmultiple)
 CFLAGS_ppc64le = -m64 -mlittle-endian -mno-vsx $(call cc-option,-mabi=elfv2)
 CFLAGS_s390 = -m64
-CFLAGS_mips32le = -EL -mabi=32
+CFLAGS_mips32le = -EL -mabi=32 -fPIC
 CFLAGS_mips32be = -EB -mabi=32
 CFLAGS_STACKPROTECTOR ?= $(call cc-option,-mstack-protector-guard=global $(call cc-option,-fstack-protector-all))
 CFLAGS  ?= -Os -fno-ident -fno-asynchronous-unwind-tables -std=c89 -W -Wall -Wextra \

---
base-commit: ba335752620565c25c3028fff9496bb8ef373602
change-id: 20770914-nolibc-pic-30043ffb0403

Best regards,
-- 
Thomas Weißschuh <linux@...ssschuh.net>