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

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

[Alexander Egorenkov]

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>
---
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);
-- 
2.31.1

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

[Philipp Rudo]

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);

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

[Heiko Carstens]

On Thu, Dec 09, 2021 at 12:01:42PM +0100, Philipp Rudo wrote:
> 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.
...
> > 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>

Applied, thanks!