Re: [PATCH v2 1/1] s390/kexec: handle R_390_PLT32DBL rela in arch_kexec_apply_relocations_add()

[Philipp Rudo <prudo@redhat.com>]

On Thu,  9 Dec 2021 08:38:17 +0100
Alexander Egorenkov <egorenar@linux.ibm.com> wrote:

> Starting with gcc 11.3, the C compiler will generate PLT-relative function
> calls even if they are local and do not require it. Later on during linking,
> the linker will replace all PLT-relative calls to local functions with
> PC-relative ones. Unfortunately, the purgatory code of kexec/kdump is
> not being linked as a regular executable or shared library would have been,
> and therefore, all PLT-relative addresses remain in the generated purgatory
> object code unresolved. This leads to the situation where the purgatory
> code is being executed during kdump with all PLT-relative addresses
> unresolved. And this results in endless loops within the purgatory code.
> 
> Furthermore, the clang C compiler has always behaved like described above
> and this commit should fix kdump for kernels built with the latter.
> 
> Because the purgatory code is no regular executable or shared library,
> contains only calls to local functions and has no PLT, all R_390_PLT32DBL
> relocation entries can be resolved just like a R_390_PC32DBL one.
> 
> * https://refspecs.linuxfoundation.org/ELF/zSeries/lzsabi0_zSeries/x1633.html#AEN1699
> 
> Relocation entries of purgatory code generated with gcc 11.3
> ------------------------------------------------------------
> 
> $ readelf -r linux/arch/s390/purgatory/purgatory.o
> 
> Relocation section '.rela.text' at offset 0x370 contains 5 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 00000000005c  000c00000013 R_390_PC32DBL     0000000000000000 purgatory_sha_regions + 2
> 00000000007a  000d00000014 R_390_PLT32DBL    0000000000000000 sha256_update + 2
> 00000000008c  000e00000014 R_390_PLT32DBL    0000000000000000 sha256_final + 2
> 000000000092  000800000013 R_390_PC32DBL     0000000000000000 .LC0 + 2
> 0000000000a0  000f00000014 R_390_PLT32DBL    0000000000000000 memcmp + 2
> 
> Relocation entries of purgatory code generated with gcc 11.2
> ------------------------------------------------------------
> 
> $ readelf -r linux/arch/s390/purgatory/purgatory.o
> 
> Relocation section '.rela.text' at offset 0x368 contains 5 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 00000000005c  000c00000013 R_390_PC32DBL     0000000000000000 purgatory_sha_regions + 2
> 00000000007a  000d00000013 R_390_PC32DBL     0000000000000000 sha256_update + 2
> 00000000008c  000e00000013 R_390_PC32DBL     0000000000000000 sha256_final + 2
> 000000000092  000800000013 R_390_PC32DBL     0000000000000000 .LC0 + 2
> 0000000000a0  000f00000013 R_390_PC32DBL     0000000000000000 memcmp + 2
> 
> Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
> Reported-by: Tao Liu <ltao@redhat.com>
> Suggested-by: Philipp Rudo <prudo@redhat.com>

Thanks!

Reviewed-by: Philipp Rudo <prudo@redhat.com>

> ---
> Changes since v1:
> =================
> * Alternative version of the fix but this time in arch_kexec_apply_relocations_add()
>   in order not to force every user of arch_kexec_do_relocs() to handle R_390_PC32DBL
>   and avoid breaking other users with unexpected bhavior 
> 
>  arch/s390/kernel/machine_kexec_file.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/s390/kernel/machine_kexec_file.c b/arch/s390/kernel/machine_kexec_file.c
> index 876cdd3c994e..8f43575a4dd3 100644
> --- a/arch/s390/kernel/machine_kexec_file.c
> +++ b/arch/s390/kernel/machine_kexec_file.c
> @@ -348,6 +348,10 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
>  		addr = section->sh_addr + relas[i].r_offset;
>  
>  		r_type = ELF64_R_TYPE(relas[i].r_info);
> +
> +		if (r_type == R_390_PLT32DBL)
> +			r_type = R_390_PC32DBL;
> +
>  		ret = arch_kexec_do_relocs(r_type, loc, val, addr);
>  		if (ret) {
>  			pr_err("Unknown rela relocation: %d\n", r_type);