Messages in this thread | | | Date | Tue, 25 Aug 2009 14:15:08 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 08/12] add trace events for each syscall entry/exit |
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* Jason Baron (jbaron@redhat.com) wrote: > 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. >
Are we certain that it will still be on the pt_regs at system call exit and not overwritten by the syscall return value ? For every arch ?
Mathieu
> thanks, > > -Jason
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