Messages in this thread | | | Date | Mon, 9 Nov 2015 11:05:36 -0500 | From | Steven Rostedt <> | Subject | Re: Compat syscall instrumentation and return from execve issue |
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On Sun, 8 Nov 2015 19:37:37 +0000 (UTC) Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> I have a few ideas on how to overcome this, and would like your > feedback on the matter: > > 1) One possible approach would be to reserve an extra status flag > in struct thread_info to get the TS_COMPAT status at syscall > entry. It would _not_ be updated when the executable is loaded, > so the state at return from execve would match the state when > entering execve. This is a simple approach, but requires kernel > changes.
Or add a flag TS_EXECVE that can be set by the tracepoint syscall enter, and checked on exit. If set, we know that the exec happened.
> > 2) Keep the compat state at system call entry in a data structure > (e.g. hash table) indexed by thread number within each tracer. > This could work around this issue within each tracer.
This is of course what you can do now. As it doesn't touch the kernel.
> > 3) Change the syscall number in the struct pt_regs whenever we > change the compat mode of a process. A 64-bit execve system > call number would be mapped to a 32-bit compat execve number, > or the opposite. This requires a kernel change, and seems to be > rather intrusive. >
This is a definite no.
I'm thinking the TS_EXECVE flag would be the least intrusive. Add a comment that it is used by tracepoints to map between compat and non-compat syscalls when execve switches the flag. This would not need to touch any of the logic of the hotpaths within the systemcalls themselves.
-- Steve
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