x86/build: Treat R_386_PLT32 relocation as R_386_PC32

This is similar to commit

  b21ebf2 ("x86: Treat R_X86_64_PLT32 as R_X86_64_PC32")

but for i386. As far as the 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.

clang-12 -fno-pic (since [1]) can emit R_386_PLT32 for compiler
generated function declarations, because preventing canonical PLT
entries is weighed over the rare ifunc diagnostic.

Further info for the more interested:

  ClangBuiltLinux/linux#1210
  https://sourceware.org/bugzilla/show_bug.cgi?id=27169
  llvm/llvm-project@a084c03 [1]

 [ bp: Massage commit message. ]

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
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>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Link: https://lkml.kernel.org/r/20210127205600.1227437-1-maskray@google.com

Author: MaskRay

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;

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;