Re: [PATCH 0/2] s390/vdso: Fix GNU_EH_FRAME PHDR

[Jens Remus <jremus@linux.ibm.com>]

On 7/10/2026 9:46 AM, Heiko Carstens wrote:
> On Mon, Jul 06, 2026 at 05:19:07PM +0200, Jens Remus wrote:
>> Patch 1 fixes the empty GNU_EH_FRAME program table header entry to
>> enable unwinders and stacktracers to locate the .eh_frame_hdr section.
>>
>> Patch 2 uses symbolic constants for the PHDR permission flags to improve
>> readability and explicitly specifies the GNU_EH_FRAME PHDR to be read-
>> only.

...

>> Jens Remus (2):
>>   s390/vdso: Pass --eh-frame-hdr to the linker
>>   s390/vdso: Use symbolic constants for the PHDR permission flags
>>
>>  arch/s390/kernel/vdso/Makefile   |  3 ++-
>>  arch/s390/kernel/vdso/vdso.lds.S | 12 ++++++++----
>>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> This series is in limbo state. Do you plan to send a new version to
> address the very minor nits, or should it stay as is?

Sorry for the delay!  I have sent a v2.

I tried hard to figure out whether adding a proper GNU_STACK PHDR
with RW in a separate commit (similar to x86 commit 8717b02b8c03
("x86/entry/vdso: Include GNU_PROPERTY and GNU_STACK PHDRs") would
provide any benefit on s390:

Loading executables in the kernel:

- s390, unlike other archs, does not provide an arch-specific
  elf_read_implies_exec(), therefore the generic from include/linux/elf.h
  that returns false is used.
- As a consequence fs/binfmt_elf.c, load_elf_binary() does not default
  executables without GNU_STACK PHDR to executable stack.

Loading shared libraries in Glibc:

- elf/dl-load.c, _dl_map_object_from_fd() does default shared libraries
  without GNU_STACK PHDR to executable stack (see DEFAULT_STACK_PERMS).


Based on that I performed the following tests, all using an executable
with GNU_STACK RW (not RWX):

Library with GNU_STACK RWX loaded at startup: [stack] rwxp (exec=YES)
Library with GNU_STACK RW  loaded at startup: [stack] rw-p (exec=NO)
Library without GNU_STACK  loaded at startup: [stack] rwxp (exec=YES)

Library with GNU_STACK RWX loaded via dlopen: fails  *1)
Library with GNU_STACK RW  loaded via dlopen: [stack] rw-p (exec=NO)
Library without GNU_STACK  loaded via dlopen: fails  *1)

vDSO without GNU_STACK loaded via startup: [stack] rw-p (exec=NO)  *2)
vDSO without GNU_STACK loaded via dlopen : [stack] rw-p (exec=NO)  *3)


*1) dlopen fails with cannot enable executable stack as shared object
    requires: Invalid argument.  Regardless of glibc.rtld.execstack=1
    tunable.

*2) Linking against /usr/modules/.../vdso/vdso.so (won't be listed in
    ldd output though)

*3) dlopen("linux-vdso.so.1", RTLD_NOW);


Technically adding a GNU_STACK PHDR with RW seems to be the right
thing, although I could not construct a use case, where it would
really matter.

> In any case:
> Acked-by: Heiko Carstens <hca@linux.ibm.com>

Thanks!

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
jremus@de.ibm.com / jremus@linux.ibm.com

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