Date: Fri, 27 Aug 2021 17:24:58 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: =?utf-8?Q?T=C4=B3l?= Coosemans <tijl@freebsd.org> Cc: Dimitry Andric <dim@freebsd.org>, freebsd-current@freebsd.org Subject: Re: i386 kernel modules unusable due to .plt sections Message-ID: <YSj1ujsZ0xQbfsnx@kib.kiev.ua> In-Reply-To: <20210827154130.7a5b141c@FreeBSD.org> References: <20210827154130.7a5b141c@FreeBSD.org>
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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
@@ -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);
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