Re: i386 kernel modules unusable due to .plt sections
- In reply to: Mark Johnston : "Re: i386 kernel modules unusable due to .plt sections"
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Date: Fri, 27 Aug 2021 16:10:24 UTC
On Fri, 27 Aug 2021 11:32:43 -0400 Mark Johnston <markj@freebsd.org> wrote: > On Fri, Aug 27, 2021 at 05:29:34PM +0200, Tijl Coosemans wrote: >> 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. > > Does disabling -zifunc-noplt fix the problem? I believe it's set by > default for i386. Removing it from sys/conf/kern.pre.mk didn't help.