From: "Dmitry V. Levin" <ldv@strace•io>
To: Sven Schnelle <svens@linux•ibm.com>
Cc: Oleg Nesterov <oleg@redhat•com>,
Eugene Syromyatnikov <evgsyr@gmail•com>,
Mike Frysinger <vapier@gentoo•org>,
Renzo Davoli <renzo@cs•unibo.it>,
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x86@kernel•org, "H. Peter Anvin" <hpa@zytor•com>,
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linux-arch@vger•kernel.org
Subject: Re: [PATCH 3/6] syscall.h: introduce syscall_set_nr()
Date: Sat, 11 Jan 2025 03:16:32 +0200 [thread overview]
Message-ID: <20250111011632.GA1724@strace.io> (raw)
In-Reply-To: <yt9dzfjz6rw5.fsf@linux.ibm.com>
On Fri, Jan 10, 2025 at 08:37:46AM +0100, Sven Schnelle wrote:
> "Dmitry V. Levin" <ldv@strace•io> writes:
>
> > Similar to syscall_set_arguments() that complements
> > syscall_get_arguments(), introduce syscall_set_nr()
> > that complements syscall_get_nr().
> >
> > syscall_set_nr() is going to be needed along with
> > syscall_set_arguments() on all HAVE_ARCH_TRACEHOOK
> > architectures to implement PTRACE_SET_SYSCALL_INFO API.
[...]
> > diff --git a/arch/s390/include/asm/syscall.h b/arch/s390/include/asm/syscall.h
> > index b3dd883699e7..1c0e349fd5c9 100644
> > --- a/arch/s390/include/asm/syscall.h
> > +++ b/arch/s390/include/asm/syscall.h
> > @@ -24,6 +24,13 @@ static inline long syscall_get_nr(struct task_struct *task,
> > (regs->int_code & 0xffff) : -1;
> > }
> >
> > +static inline void syscall_set_nr(struct task_struct *task,
> > + struct pt_regs *regs,
> > + int nr)
> > +{
>
> I think there should be a
>
> if (!test_pt_regs_flags(regs, PIF_SYSCALL))
> return;
>
> before the modification so a user can't accidentally change int_code
> when ptrace stopped in a non-syscall path.
The reason why syscall_get_nr() has this check on s390 (and similar checks
on arc, powerpc, and sparc) is that syscall_get_nr() can be called while
the target task is not in syscall.
Unlike syscall_get_nr(), syscall_set_nr() can be called only when the
target task is stopped for tracing on entering syscall: the description in
include/asm-generic/syscall.h explicitly states that, and the follow-up
patch that introduces PTRACE_SET_SYSCALL_INFO adds a syscall_set_nr() call
when the tracee is stopped on entering syscall in either
PTRACE_SYSCALL_INFO_ENTRY or PTRACE_SYSCALL_INFO_SECCOMP state.
I don't mind adding a check, but syscall_set_nr() invocation while the
target task is not in syscall wouldn't be a result of user actions but
a kernel programing error, and in that case WARN_ON_ONCE() would be more
appropriate.
If calling syscall_set_nr() while the target task is not in syscall was
legal, then syscall_set_nr() would have been designed to return a value
indicating the status of operation.
Anyway, I'll add an explanatory comment to syscall_set_nr() on all
architectures where syscall_get_nr() has a check.
--
ldv
next prev parent reply other threads:[~2025-01-11 1:16 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-01-07 23:01 [PATCH 0/6] ptrace: introduce PTRACE_SET_SYSCALL_INFO API Dmitry V. Levin
2025-01-07 23:04 ` [PATCH 1/6] Revert "arch: remove unused function syscall_set_arguments()" Dmitry V. Levin
2025-01-07 23:04 ` [PATCH 3/6] syscall.h: introduce syscall_set_nr() Dmitry V. Levin
2025-01-10 7:37 ` Sven Schnelle
2025-01-11 1:16 ` Dmitry V. Levin [this message]
2025-01-10 3:15 ` [PATCH 0/6] ptrace: introduce PTRACE_SET_SYSCALL_INFO API H. Peter Anvin
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