Re: i386 kernel modules unusable due to .plt sections

On Fri, 27 Aug 2021 17:24:58 +0300 Konstantin Belousov
<kostikbel@gmail.com> wrote:
> On Fri, Aug 27, 2021 at 03:41:30PM +0200, Tijl Coosemans wrote:
>> 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
> @@ -245,6 +245,7 @@ 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);

No, I've tried that.  It handles the relocation but callers still don't
setup %ebx as PIC register.  I'm looking for someone to confirm it's a
compiler bug and not a missing flag or something.  I tried -fno-plt and
that has no effect.