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    SubjectRe: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat
    On Fri, Apr 16, 2021 at 02:34:41PM +0100, Mark Rutland wrote:
    > On Fri, Apr 16, 2021 at 01:33:22PM +0100, 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@windriver.com>
    > > > ---
    > > > 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.
    > >
    > > 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.
    >
    > I think this is a problem we created for ourselves back in commit:
    >
    > 15956689a0e60aa0 ("arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return)
    >
    > AFAICT, the perf regs samples are the only place this matters, since for
    > ptrace the compat regs are implicitly truncated to compat_ulong_t, and
    > audit expects the non-truncated return value. Other architectures don't
    > truncate here, so I think we're setting ourselves up for a game of
    > whack-a-mole to truncate and extend wherever we need to.
    >
    > Given that, I suspect it'd be better to do something like the below.
    >
    > Will, thoughts?

    I think perf is one example, but this is also visible to userspace via the
    native ptrace interface and I distinctly remember needing this for some
    versions of arm64 strace to work correctly when tracing compat tasks.

    So I do think that clearing the upper bits on the return path is the right
    approach, but it sounds like we need some more work to handle syscall(-1)
    and audit (what exactly is the problem here after these patches have been
    applied?)

    Will

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