Re: [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(a)windriver.com&gt; >>> --- >>> 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