[PATCH v4] x86: Treat R_386_PLT32 as R_386_PC32

[Fangrui Song <maskray@google.com>]

This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as
R_X86_64_PC32"), but for i386.  As far as Linux kernel is concerned,
R_386_PLT32 can be treated the same as R_386_PC32.

R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types which can
only be used by branches. If the referenced symbol is defined
externally, a PLT will be used.
R_386_PC32/R_X86_64_PC32 are PC-relative relocation types which can be
used by address taking operations and branches. If the referenced symbol
is defined externally, a copy relocation/canonical PLT entry will be
created in the executable.

On x86-64, there is no PIC vs non-PIC PLT distinction and an
R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
`call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.
This avoids canonical PLT entries (st_shndx=0, st_value!=0).

On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
GCC/GNU as convention is to use R_386_PC32 for non-PIC PLT and
R_386_PLT32 for PIC PLT. Copy relocations/canonical PLT entries are
possible ABI issues but GCC/GNU as will likely keep the status quo
because (1) the ABI is legacy (2) the change will drop a GNU ld
diagnostic for non-default visibility ifunc in shared objects.
https://sourceware.org/bugzilla/show_bug.cgi?id=27169

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
can emit R_386_PLT32 for compiler generated function declarations,
because preventing canonical PLT entries is weighed over the rare ifunc
diagnostic.

Link: https://github.com/ClangBuiltLinux/linux/issues/1210
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>

---
Change in v2:
* Improve commit message
---
Change in v3:
* Change the GCC link to the more relevant GNU as link.
* Fix the relevant llvm-project commit.
---
Change in v4:
* Improve comments and commit message
---
 arch/x86/kernel/module.c |  1 +
 arch/x86/tools/relocs.c  | 12 ++++++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 34b153cbd4ac..5e9a34b5bd74 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -114,6 +114,7 @@ int apply_relocate(Elf32_Shdr *sechdrs,
 			*location += sym->st_value;
 			break;
 		case R_386_PC32:
+		case R_386_PLT32:
 			/* Add the value, subtract its position */
 			*location += sym->st_value - (uint32_t)location;
 			break;
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index ce7188cbdae5..1c3a1962cade 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -867,9 +867,11 @@ static int do_reloc32(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
 	case R_386_PC32:
 	case R_386_PC16:
 	case R_386_PC8:
+	case R_386_PLT32:
 		/*
-		 * NONE can be ignored and PC relative relocations don't
-		 * need to be adjusted.
+		 * NONE can be ignored and PC relative relocations don't need
+		 * to be adjusted. Because sym must be defined, R_386_PLT32 can
+		 * be treated the same way as R_386_PC32.
 		 */
 		break;
 
@@ -910,9 +912,11 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
 	case R_386_PC32:
 	case R_386_PC16:
 	case R_386_PC8:
+	case R_386_PLT32:
 		/*
-		 * NONE can be ignored and PC relative relocations don't
-		 * need to be adjusted.
+		 * NONE can be ignored and PC relative relocations don't need
+		 * to be adjusted. Because sym must be defined, R_386_PLT32 can
+		 * be treated the same way as R_386_PC32.
 		 */
 		break;
 
-- 
2.30.0.280.ga3ce27912f-goog