Re: [PATCH] i386/audit: stop scribbling on the stack frame

[Richard Guy Briggs <rgb@redhat.com>]

On 14/10/27, Eric Paris wrote:
> My patch was already committed to the -tip urgent branch.  I believe any
> optimization should be based on that branch, Richard.  If you are trying
> to wrangle every bit of speed out of this, should you
> 
> push %esi;
> push %edi;
> CFI_ADJUST_CFA_OFFSET 8
> call __audit_syscall_entry
> pop;
> pop;
> CFI_ADJUST_CFA_OFFSET -8
> 
> Instead of using the pushl_cfi and popl_cfi macros?
> 
> I wrote my patch to be obviously correct, but agree there are certainly
> some speedups possible.

I was just more surprised that you didn't use the assumptions and
approach of the original code before I botched it, which simply re-used
values in registers that were already there, rather than fetching them
from the pt_regs struct on the stack and adjusting the reference with an
offset on the fly as you're pushing items onto the stack.

> -Eric
> 
> On Sun, 2014-10-26 at 22:34 -0400, Richard Guy Briggs wrote:
> > git commit b4f0d3755c5e9cc86292d5fd78261903b4f23d4a was very very dumb.
> > It was writing over %esp/pt_regs semi-randomly on i686 with the expected
> > "system can't boot" results.  As noted in:
> > 
> > https://bugs.freedesktop.org/show_bug.cgi?id=85277
> > 
> > This patch stops fscking with pt_regs.  Instead it sets up the registers
> > for the call to __audit_syscall_entry in the most obvious conceivable
> > way.  It then does just a tiny tiny touch of magic.  We need to get what
> > started in PT_EDX into 0(%esp) and PT_ESI into 4(%esp).  This is as easy
> > as a pair of pushes using the values still in those registers.
> > 
> > After the call to __audit_syscall_entry all we need to do is get that
> > now useless junk off the stack (pair of pops) and reload %eax with the
> > original syscall so other stuff can keep going about it's business.
> > 
> > Reported-by: Paulo Zanoni <przanoni@gmail.com>
> > Signed-off-by: Eric Paris <eparis@redhat.com>
> > Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: x86@kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: linux-audit@redhat.com
> > 
> > ---
> > On 14/10/25, Thomas Gleixner wrote:
> > > Why are we grabbing that from the stack? AFAICT all arguments are in
> > > the registers still.
> > 
> > Right, re-arranging the instructions slightly to avoid overwriting %edx
> > with %ebx before needing it to push onto the stack, how does this look?
> > 
> >  arch/x86/kernel/entry_32.S | 15 +++++++--------
> >  1 file changed, 7 insertions(+), 8 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
> > index b553ed8..344b63f 100644
> > --- a/arch/x86/kernel/entry_32.S
> > +++ b/arch/x86/kernel/entry_32.S
> > @@ -447,15 +447,14 @@ sysenter_exit:
> >  sysenter_audit:
> >  	testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
> >  	jnz syscall_trace_entry
> > -	addl $4,%esp
> > -	CFI_ADJUST_CFA_OFFSET -4
> > -	movl %esi,4(%esp)		/* 5th arg: 4th syscall arg */
> > -	movl %edx,(%esp)		/* 4th arg: 3rd syscall arg */
> > -	/* %ecx already in %ecx		   3rd arg: 2nd syscall arg */
> > -	movl %ebx,%edx			/* 2nd arg: 1st syscall arg */
> > -	/* %eax already in %eax		   1st arg: syscall number */
> > +	/* movl PT_ECX(%esp), %ecx	already set, a1: 3nd arg to audit */
> > +	/* movl PT_EAX(%esp), %eax	already set, syscall number: 1st arg to audit */
> > +	pushl_cfi %esi			/* a3: 5th arg */
> > +	pushl_cfi %edx			/* a2: 4th arg */
> > +	movl %ebx, %edx			/* ebx/a0: 2nd arg to audit */
> >  	call __audit_syscall_entry
> > -	pushl_cfi %ebx
> > +	popl_cfi %ecx /* get that remapped edx off the stack */
> > +	popl_cfi %ecx /* get that remapped esi off the stack */
> >  	movl PT_EAX(%esp),%eax		/* reload syscall number */
> >  	jmp sysenter_do_call
> >  
> > 
> > - RGB
> > 
> > --
> > Richard Guy Briggs <rbriggs@redhat.com>
> > Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
> > Remote, Ottawa, Canada
> > Voice: +1.647.777.2635, Internal: (81) 32635, Alt: +1.613.693.0684x3545
> 
> 

- RGB

--
Richard Guy Briggs <rbriggs@redhat.com>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635, Alt: +1.613.693.0684x3545