Re: i386 kernel modules unusable due to .plt sections

From: Konstantin Belousov <kostikbel_at_gmail.com>
Date: Fri, 27 Aug 2021 17:24:58 +0300
On Fri, Aug 27, 2021 at 03:41:30PM +0200, Tijl Coosemans wrote:
> Hi,
> 
> I use devel/llvm* to build base and just switched to llvm12.  It seems
> that on i386 clang12 uses R_386_PLT32 relocations for some calls to at
> least memset, memcpy and __stack_chk_fail (clang11 uses R_386_PC32).
> These are converted to R_386_JMP_SLOT relocations by the linker which
> aren't supported by the kernel, e.g. loading linux.ko gives "kldload:
> unexpected relocation type" from sys/i386/i386/elf_machdep.c.  The PLT
> entries also depend on a base pointer in %ebx but kernel modules aren't
> compiled with -fPIC, so this can't work and I suspect this is a
> regression in clang12.
> 
> The following code shows the difference between clang11 and clang12:
> 
> --------
> #include <string.h>
> 
> void *
> test_memset(void *p, int c, size_t len) {
>         return (memset(p, c, len));
> }
> 
> void *
> test_memcpy(void *dst, const void *src, size_t len) {
>         return (memcpy(dst, src, len));
> }
> 
> void *
> test_memmove(void *dst, const void *src, size_t len) {
>         return (memmove(dst, src, len));
> }
> --------
> 
> Output of "readelf -r test.o" when compiled with "clang12 -c test.c -m32":
> r_offset r_info   r_type              st_value st_name
> 0000002c 00000504 R_386_PLT32         00000000 memset
> 00000067 00000304 R_386_PLT32         00000000 memcpy
> 000000a7 00000402 R_386_PC32          00000000 memmove
> 
> With clang11:
> r_offset r_info   r_type              st_value st_name
> 00000036 00000502 R_386_PC32          00000000 memset
> 00000083 00000302 R_386_PC32          00000000 memcpy
> 000000d2 00000402 R_386_PC32          00000000 memmove

Are you asking (for somebody) to add R_386_JMP_SLOT to i386/elf_machdep.c?
Like this, not even built.

diff --git a/sys/i386/i386/elf_machdep.c b/sys/i386/i386/elf_machdep.c
index 3754b36d9e33..a26a4189e0ee 100644
--- a/sys/i386/i386/elf_machdep.c
+++ b/sys/i386/i386/elf_machdep.c
_at__at_ -245,6 +245,7 _at__at_ elf_reloc_internal(linker_file_t lf, Elf_Addr relocbase, const void *data,
 			break;
 
 		case R_386_GLOB_DAT:	/* S */
+		case R_386_JMP_SLOT:
 			error = lookup(lf, symidx, 1, &addr);
 			if (error != 0)
 				return (-1);
Received on Fri Aug 27 2021 - 14:24:58 UTC

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