linux-kernel - Re: ia32_sysenter_target does not preserve EFLAGS

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Date:	Fri, 27 Mar 2015 13:00:59 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: ia32_sysenter_target does not preserve EFLAGS

On Fri, Mar 27, 2015 at 7:25 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>
> Apparently, users *don't* depend on arithmetic flags
> to survive over syscall. They also okay with DF flag
> being cleared.

Generally, users probably dont' care about many registers at all being
saved, but it's worth noting that the reason system calls save/restore
even caller-saved registers is at least partly in order to avoid any
kernel information leaks.

I don't believe that user mode will ever reasonably care about the
arithmetic flags being changed, but at the same time I also don't it
is something we should ever consider a "feature" we should try to take
advantage of. Generally we should try to not mess with the flag state,
and I'd *much* rather make the rule be that all the system call return
paths restore flags as much as possible.

                          Linus
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