linux-kernel - [PATCH 2/7] modpost: fix section mismatch message for R_ARM

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Message-Id: <20230601121001.1071533-3-masahiroy@kernel.org>
Date:   Thu,  1 Jun 2023 21:09:56 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        Russell King <rmk+kernel@....linux.org.uk>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Sam Ravnborg <sam@...nborg.org>,
        Tony Lindgren <tony@...mide.com>
Subject: [PATCH 2/7] modpost: fix section mismatch message for R_ARM_{PC24,CALL,JUMP24}

addend_arm_rel() processes R_ARM_PC24, R_ARM_CALL, R_ARM_JUMP24 in a
wrong way.

Here, test code.

[test code for R_ARM_JUMP24]

  .section .init.text,"ax"
  bar:
          bx      lr

  .section .text,"ax"
  .globl foo
  foo:
          b       bar

[test code for R_ARM_CALL]

  .section .init.text,"ax"
  bar:
          bx      lr

  .section .text,"ax"
  .globl foo
  foo:
          push    {lr}
          bl      bar
          pop     {pc}

If you compile it with ARM multi_v7_defconfig, modpost will show the
symbol name, (unknown).

  WARNING: modpost: vmlinux.o: section mismatch in reference: foo (section: .text) -> (unknown) (section: .init.text)

(You need to use GNU linker instead of LLD to reproduce it.)

Fix the code to make modpost show the correct symbol name.

I imported (with adjustment) sign_extend32() from include/linux/bitops.h.

The '+8' is the compensation for pc-relative instruction. It is
documented in "ELF for the Arm Architecture" [1].

  "If the relocation is pc-relative then compensation for the PC bias
  (the PC value is 8 bytes ahead of the executing instruction in Arm
  state and 4 bytes in Thumb state) must be encoded in the relocation
  by the object producer."

[1]: https://github.com/ARM-software/abi-aa/blob/main/aaelf32/aaelf32.rst

Fixes: 56a974fa2d59 ("kbuild: make better section mismatch reports on arm")
Fixes: 6e2e340b59d2 ("ARM: 7324/1: modpost: Fix section warnings for ARM for many compilers")
Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 scripts/mod/modpost.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index c68dad45ace2..e47bba7cfad2 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1277,12 +1277,20 @@ static int addend_386_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
 #define	R_ARM_THM_JUMP19	51
 #endif
 
+static int32_t sign_extend32(int32_t value, int index)
+{
+	uint8_t shift = 31 - index;
+
+	return (int32_t)(value << shift) >> shift;
+}
+
 static int addend_arm_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
 {
 	unsigned int r_typ = ELF_R_TYPE(r->r_info);
 	Elf_Sym *sym = elf->symtab_start + ELF_R_SYM(r->r_info);
 	void *loc = reloc_location(elf, sechdr, r);
 	uint32_t inst;
+	int32_t offset;
 
 	switch (r_typ) {
 	case R_ARM_ABS32:
@@ -1292,6 +1300,10 @@ static int addend_arm_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
 	case R_ARM_PC24:
 	case R_ARM_CALL:
 	case R_ARM_JUMP24:
+		inst = TO_NATIVE(*(uint32_t *)loc);
+		offset = sign_extend32((inst & 0x00ffffff) << 2, 25);
+		r->r_addend = offset + sym->st_value + 8;
+		break;
 	case R_ARM_THM_CALL:
 	case R_ARM_THM_JUMP24:
 	case R_ARM_THM_JUMP19:
-- 
2.39.2