linux-kernel - Re: [PATCH -v2] Audit: push audit success and retcode into arch ptrace.h

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Date:	Tue, 07 Jun 2011 14:53:11 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, tony.luck@...el.com,
	fenghua.yu@...el.com, monstr@...str.eu, ralf@...ux-mips.org,
	benh@...nel.crashing.org, paulus@...ba.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, linux390@...ibm.com,
	lethal@...ux-sh.org, davem@...emloft.net, jdike@...toit.com,
	richard@....at, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, linux-ia64@...r.kernel.org,
	microblaze-uclinux@...e.uq.edu.au, linux-mips@...ux-mips.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH -v2] Audit: push audit success and retcode into arch
 ptrace.h

On Tue, 2011-06-07 at 19:19 +0200, Oleg Nesterov wrote:
> On 06/03, Eric Paris wrote:
> >
> > The audit system previously expected arches calling to audit_syscall_exit to
> > supply as arguments if the syscall was a success and what the return code was.
> > Audit also provides a helper AUDITSC_RESULT which was supposed to simplify things
> > by converting from negative retcodes to an audit internal magic value stating
> > success or failure.  This helper was wrong and could indicate that a valid
> > pointer returned to userspace was a failed syscall.  The fix is to fix the
> > layering foolishness.  We now pass audit_syscall_exit a struct pt_reg and it
> > in turns calls back into arch code to collect the return value and to
> > determine if the syscall was a success or failure.  We also define a generic
> > is_syscall_success() macro which determines success/failure based on if the
> > value is < -MAX_ERRNO.  This works for arches like x86 which do not use a
> > separate mechanism to indicate syscall failure.
> 
> I know nothing about audit, but the patch looks fine to me.
> 
> 
> But I have a bit off-topic question,
> 
> > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> > index 8a445a0..b7b1f88 100644
> > --- a/arch/x86/kernel/entry_64.S
> > +++ b/arch/x86/kernel/entry_64.S
> > @@ -53,6 +53,7 @@
> >  #include <asm/paravirt.h>
> >  #include <asm/ftrace.h>
> >  #include <asm/percpu.h>
> > +#include <linux/err.h>
> >
> >  /* Avoid __ASSEMBLER__'ifying <linux/audit.h> just for this.  */
> >  #include <linux/elf-em.h>
> > @@ -564,17 +565,16 @@ auditsys:
> >  	jmp system_call_fastpath
> >
> >  	/*
> > -	 * Return fast path for syscall audit.  Call audit_syscall_exit()
> > +	 * Return fast path for syscall audit.  Call __audit_syscall_exit()
> >  	 * directly and then jump back to the fast path with TIF_SYSCALL_AUDIT
> >  	 * masked off.
> >  	 */
> >  sysret_audit:
> >  	movq RAX-ARGOFFSET(%rsp),%rsi	/* second arg, syscall return value */
> > -	cmpq $0,%rsi		/* is it < 0? */
> > -	setl %al		/* 1 if so, 0 if not */
> > +	cmpq $-MAX_ERRNO,%rsi	/* is it < -MAX_ERRNO? */
> > +	setbe %al		/* 1 if so, 0 if not */
> >  	movzbl %al,%edi		/* zero-extend that into %edi */
> > -	inc %edi /* first arg, 0->1(AUDITSC_SUCCESS), 1->2(AUDITSC_FAILURE) */
> > -	call audit_syscall_exit
> > +	call __audit_syscall_exit
> 
> With or without this patch, can't we call audit_syscall_exit() twice
> if there is something else in _TIF_WORK_SYSCALL_EXIT mask apart from
> SYSCALL_AUDIT ? First time it is called from asm, then from
> syscall_trace_leave(), no?
> 
> For example. The task has TIF_SYSCALL_AUDIT and nothing else, it does
> system_call->auditsys->system_call_fastpath. What if it gets, say,
> TIF_SYSCALL_TRACE before ret_from_sys_call?

No harm is done calling twice.  The first call will do the real work and
cleanup.  It will set a flag in the audit data that the work has been
done (in_syscall == 0) thus the second call will then not do any real
work and won't have anything to clean up.

-Eric

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