Date: Fri, 27 Aug 2021 17:29:34 +0200 From: =?UTF-8?B?VMSzbA==?= Coosemans <tijl@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Dimitry Andric <dim@freebsd.org>, freebsd-current@freebsd.org Subject: Re: i386 kernel modules unusable due to .plt sections Message-ID: <20210827172934.41e3a3f4@FreeBSD.org> In-Reply-To: <YSj1ujsZ0xQbfsnx@kib.kiev.ua> References: <20210827154130.7a5b141c@FreeBSD.org> <YSj1ujsZ0xQbfsnx@kib.kiev.ua>
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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, T=C4=B3l 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.
>>=20
>> The following code shows the difference between clang11 and clang12:
>>=20
>> --------
>> #include <string.h>
>>=20
>> void *
>> test_memset(void *p, int c, size_t len) {
>> return (memset(p, c, len));
>> }
>>=20
>> void *
>> test_memcpy(void *dst, const void *src, size_t len) {
>> return (memcpy(dst, src, len));
>> }
>>=20
>> void *
>> test_memmove(void *dst, const void *src, size_t len) {
>> return (memmove(dst, src, len));
>> }
>> --------
>>=20
>> 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
>>=20
>> 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 =20
>=20
> Are you asking (for somebody) to add R_386_JMP_SLOT to i386/elf_machdep.c?
> Like this, not even built.
>=20
> 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 relocba=
se, const void *data,
> break;
> =20
> case R_386_GLOB_DAT: /* S */
> + case R_386_JMP_SLOT:
> error =3D lookup(lf, symidx, 1, &addr);
> if (error !=3D 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.
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