Re: [PATCH] signal: optimize 'sigaction' call path
From: Oleg Nesterov
Date: Tue Apr 07 2015 - 06:06:54 EST
> There is a set of syscalls in the kernel about 'sigaction'.
> All they end up with calling the helper 'do_sigaction',
> so the generic scheme is:
> - copy user data to kernel;
> - 'do_sigaction';
> - copy kernel data to user.
> 'do_sigaction' checks 'signum' parameter before doing its main job.
> If this check fails syscall fails immediately, as well. But at this
> stage first copy is already done. And so there's a potential chance
> having it useless. It may affect performance significantly if user
> data was, say, swapped, and a fault was handled to obtain it.
Only if the signal number is wrong? So why do we care?
> In this patch, 'signum' sanity check is moved out of 'do_sigaction'
> to a small function 'user_signal'. So we can call it before any copying.
> arch/alpha/kernel/signal.c | 19 +++++++-------
> arch/mips/kernel/signal.c | 10 +++++---
> arch/mips/kernel/signal32.c | 10 +++++---
> arch/sparc/kernel/sys_sparc32.c | 10 ++++----
> arch/sparc/kernel/sys_sparc_32.c | 10 ++++----
> arch/sparc/kernel/sys_sparc_64.c | 10 ++++----
> include/linux/sched.h | 2 +-
> include/linux/signal.h | 5 ++++
> kernel/signal.c | 54 +++++++++++++++++++++-------------------
> 9 files changed, 71 insertions(+), 59 deletions(-)
And this blows the source and compiled code. Not too much, but this
change should be justified somehow.
And to me this patch doesn't look like a cleanup, imho this sanity
check makes more sense in one place.
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