[PATCH] arm: always update thread_info->syscall

Russell King - ARM Linux
2018-11-26T23:44:56Z

On Mon, Nov 26, 2018 at 11:41:11PM +0000, Russell King - ARM Linux wrote:
> On Mon, Nov 26, 2018 at 11:33:03PM +0000, Russell King - ARM Linux wrote:
> > On Mon, Nov 26, 2018 at 08:53:35PM -0200, Rafael David Tinoco wrote:
> > > Right now, only way for task->thread_info->syscall to be updated is if
> > > if _TIF_SYSCALL_WORK is set in current's task thread_info->flags
> > > (similar to what has_syscall_work() checks for arm64).
> > >
> > > This means that "->syscall" will only be updated if we are tracing the
> > > syscalls through ptrace, for example. This is NOT the same behavior as
> > > arm64, when pt_regs->syscallno is updated in the beginning of svc0
> > > handler for *every* syscall entry.
> >
> > So when was it decided that the syscall number will always be required
> > (we need it to know how far back this has to be backported).
>
> PS, I rather object to the fact that the required behaviour seems to
> change, arch maintainers aren't told about it until... some test is
> created at some random point in the future which then fails.
>
> Surely there's a better way to communicate changes in requirements
> than discovery-by-random-bug-report ?

Final comment for tonight - the commit introducing /proc/*/syscall says:

This adds /proc/PID/syscall and /proc/PID/task/TID/syscall magic files.
These use task_current_syscall() to show the task's current system call
number and argument registers, stack pointer and PC. For a task blocked
but not in a syscall, the file shows "-1" in place of the syscall number,
followed by only the SP and PC. For a task that's not blocked, it shows
"running".

Please validate that a blocked task does indeed show -1 with your patch
applied.

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