Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat

[Mark Rutland]

On Tue, Apr 20, 2021 at 04:42:53PM +0800, He Zhe wrote:
> On 4/16/21 8:33 PM, Catalin Marinas wrote:
> > On Fri, Apr 16, 2021 at 03:55:31PM +0800, He Zhe wrote:
> >> The general version of is_syscall_success does not handle 32-bit
> >> compatible case, which would cause 32-bit negative return code to be
> >> recoganized as a positive number later and seen as a "success".
> >>
> >> Since is_compat_thread is defined in compat.h, implementing
> >> is_syscall_success in ptrace.h would introduce build failure due to
> >> recursive inclusion of some basic headers like mutex.h. We put the
> >> implementation to ptrace.c
> >>
> >> Signed-off-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
> >> ---
> >>  arch/arm64/include/asm/ptrace.h |  3 +++
> >>  arch/arm64/kernel/ptrace.c      | 10 ++++++++++
> >>  2 files changed, 13 insertions(+)
> >>
> >> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> >> index e58bca832dff..3c415e9e5d85 100644
> >> --- a/arch/arm64/include/asm/ptrace.h
> >> +++ b/arch/arm64/include/asm/ptrace.h
> >> @@ -328,6 +328,9 @@ static inline void regs_set_return_value(struct pt_regs *regs, unsigned long rc)
> >>  	regs->regs[0] = rc;
> >>  }
> >>  
> >> +extern inline int is_syscall_success(struct pt_regs *regs);
> >> +#define is_syscall_success(regs) is_syscall_success(regs)
> >> +
> >>  /**
> >>   * regs_get_kernel_argument() - get Nth function argument in kernel
> >>   * @regs:	pt_regs of that context
> >> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> >> index 170f42fd6101..3266201f8c60 100644
> >> --- a/arch/arm64/kernel/ptrace.c
> >> +++ b/arch/arm64/kernel/ptrace.c
> >> @@ -1909,3 +1909,13 @@ int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task)
> >>  	else
> >>  		return valid_native_regs(regs);
> >>  }
> >> +
> >> +inline int is_syscall_success(struct pt_regs *regs)
> >> +{
> >> +	unsigned long val = regs->regs[0];
> >> +
> >> +	if (is_compat_thread(task_thread_info(current)))
> >> +		val = sign_extend64(val, 31);
> >> +
> >> +	return !IS_ERR_VALUE(val);
> >> +}
> > It's better to use compat_user_mode(regs) here instead of
> > is_compat_thread(). It saves us from worrying whether regs are for the
> > current context.
> 
> Thanks. I'll use this for v2.
> 
> >
> > I think we should change regs_return_value() instead. This function
> > seems to be called from several other places and it has the same
> > potential problems if called on compat pt_regs.
> 
> IMHO, now that we have had specific function, syscall_get_return_value, to get
> syscall return code, we might as well use it. regs_return_value may be left for
> where we want internal return code. I found such places below and haven't found
> other places that syscall sign extension is concerned about.
> 
> kernel/test_kprobes.c
> kernel/trace/trace_kprobe.c
> samples/kprobes/kretprobe_example.c

FWIW, I agree that we should use syscall_get_return_value(). If we make
the common implementation of is_syscall_success() use
syscall_get_return_value(), we shouldn't need to write our own
implementation, so I'd prefer if we could do that if possible.

IIUC regs_get_return_value() was originally meant to be used for kernel
regs, and is trying to do something quite different, so having the core
code use syscall_get_return_value() makes sense to allow architectures
to handle those cases differently.

Thanks,
Mark.