Re: [RFC] ptrace: add generic SET_SYSCALL request

[AKASHI Takahiro]

On 11/12/2014 08:19 PM, Arnd Bergmann wrote:
> On Wednesday 12 November 2014 11:13:52 Will Deacon wrote:
> > On Wed, Nov 12, 2014 at 11:06:59AM +0000, AKASHI Takahiro wrote:
> > > On 11/12/2014 08:00 PM, Will Deacon wrote:
> > > > On Wed, Nov 12, 2014 at 10:46:01AM +0000, AKASHI Takahiro wrote:
> > > > > On 11/07/2014 11:04 PM, Oleg Nesterov wrote:
> > > > > > To me the fact that PTRACE_SET_SYSCALL can be undefined and syscall_set_nr()
> > > > > > is very much arch-dependant (but most probably trivial) means that this  code
> > > > > > should live in arch_ptrace().
> > > > >
> > > > > Thinking of Oleg's comment above, it doesn't make sense neither to define generic
> > > > > NT_SYSTEM_CALL (user_regset) in uapi/linux/elf.h and implement it in ptrace_regset()
> > > > > in kernel/ptrace.c with arch-defined syscall_(g)set_nr().
> > > > >
> > > > > Since we should have the same interface on arm and arm64, we'd better implement
> > > > > ptrace(PTRACE_SET_SYSCALL) locally on arm64 for now (as I originally submitted).
> > > >
> > > > I think the regset approach is cleaner. We already do something similar for
> > > > TLS. That would be implemented under arch/arm64/ with it's own NT type.
> > >
> > > Okey, so arm64 goes its own way
> > > Or do you want to have a similar regset on arm, too?
> > > (In this case, NT_ARM_SYSTEM_CALL can be shared in uapi/linux/elf.h)
> >
> > Just do arm64. We already have the dedicated request for arch/arm/.
>
> I wonder if we should define NT_ARM64_SYSTEM_CALL to the same value
> as NT_S390_SYSTEM_CALL (0x307), or even define it as an architecture-
> independent NT_SYSTEM_CALL number with that value, so other architectures
> don't have to introduce new types when they also want to implement it.

I digged into gdb code (gdb/bfd/elf.c):
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=bfd/elf.c;h=8b207ad872a3992381e93bdfa0a75ef444651613;hb=HEAD
  elf_parse_notes()->elfcore_grok_note()->elfcore_grok_s390_system_call()

It seems to me that NT_S390_SYSTEM_CALL(=0x307) is recognized as a s390 specific
value (without checking for machine type). So thinking of potential conflict, it might not be
a good idea to use this value as a common number (of NT_SYSTEM_CALL).
It's very unlikely that a "note" section for NT_(S390_)SYSTEM_CALL appears in a coredump file, though.

What do you think?

-Takahiro AKASHI

> 	Arnd
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