Re: [PATCH 08/12] add trace events for each syscall entry/exit

From: Jason Baron
Date: Tue Aug 25 2009 - 13:06:06 EST

On Tue, Aug 25, 2009 at 12:20:04PM -0400, Mathieu Desnoyers wrote:
> Uh ? kernel threads can invoke a system call. There are rare places
> where kernel code actually invoke system calls. I don't see why we
> should not deal with them.
> Moreover, the problem you face is more general: if we set the
> TIF_SYSCALL_FTRACE flag of a standard thread right in the middle of its
> system call, x86_64 will cause the syscall exit to execute by re-reading
> the thread flags and run a syscall trace exit.
> We could simply initialize the "saved system calls id" number to
> something like -1, so that if we happen to return from a syscall that
> did not get its id recorded at syscall entry, we know it because it's
> not initialized.
> We would need to carefully put back the -1 value after clearing the
> thread flag when we stop tracing too (while still holding a mutex).
> Mathieu

why can't we have a syscall exit that is unmatched? we calculate
the exit syscall number for the the pt_regs structure at exit, so we
don't need to match it up with an entry to know which syscall it is.


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