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SubjectRe: [PATCH 09/11] x86/fault: Rename no_context() to kernelmode_fixup_or_oops()
On Wed, Feb 03, 2021 at 11:53:03AM -0800, Andy Lutomirski wrote:
> I feel like that would be more obfuscated — then the function would
> return without fixing anything for usermode faults, return after
> fixing it for kernel mode faults, or oops.

You practically pretty much have it already with the WARN_ON_ONCE. And
you can make the thing return 1 to denote it was in user_mode() and 0
otherwise. IINMSO, something like this:

---
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 08f5f74cf989..2b86d541b181 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -692,11 +692,12 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code,
oops_end(flags, regs, sig);
}

-static noinline void
+static noinline int
kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
unsigned long address, int signal, int si_code)
{
- WARN_ON_ONCE(user_mode(regs));
+ if (WARN_ON_ONCE(user_mode(regs)))
+ return 1;

/* Are we prepared to handle this kernel fault? */
if (fixup_exception(regs, X86_TRAP_PF, error_code, address)) {
@@ -706,7 +707,7 @@ kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
* task context.
*/
if (in_interrupt())
- return;
+ return 0;

/*
* Per the above we're !in_interrupt(), aka. task context.
@@ -726,7 +727,7 @@ kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
/*
* Barring that, we can do the fixup and be happy.
*/
- return;
+ return 0;
}

/*
@@ -734,9 +735,11 @@ kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
* instruction.
*/
if (is_prefetch(regs, error_code, address))
- return;
+ return 0;

page_fault_oops(regs, error_code, address);
+
+ return 0;
}

/*
@@ -781,10 +784,8 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
{
struct task_struct *tsk = current;

- if (!user_mode(regs)) {
- kernelmode_fixup_or_oops(regs, error_code, address, pkey, si_code);
+ if (!kernelmode_fixup_or_oops(regs, error_code, address, pkey, si_code))
return;
- }

if (!(error_code & X86_PF_USER)) {
/* Implicit user access to kernel memory -- just oops */
@@ -913,10 +914,8 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
vm_fault_t fault)
{
/* Kernel mode? Handle exceptions or die: */
- if (!user_mode(regs)) {
- kernelmode_fixup_or_oops(regs, error_code, address, SIGBUS, BUS_ADRERR);
+ if (!kernelmode_fixup_or_oops(regs, error_code, address, SIGBUS, BUS_ADRERR))
return;
- }

/* User-space => ok to do another page fault: */
if (is_prefetch(regs, error_code, address))
@@ -1378,10 +1377,8 @@ void do_user_addr_fault(struct pt_regs *regs,
* Quick path to respond to signals. The core mm code
* has unlocked the mm for us if we get here.
*/
- if (!user_mode(regs))
- kernelmode_fixup_or_oops(regs, error_code, address,
- SIGBUS, BUS_ADRERR);
- return;
+ if (!kernelmode_fixup_or_oops(regs, error_code, address, SIGBUS, BUS_ADRERR))
+ return;
}

/*
@@ -1399,18 +1396,15 @@ void do_user_addr_fault(struct pt_regs *regs,
if (likely(!(fault & VM_FAULT_ERROR)))
return;

- if (fatal_signal_pending(current) && !user_mode(regs)) {
- kernelmode_fixup_or_oops(regs, error_code, address, 0, 0);
- return;
+ if (fatal_signal_pending(current)) {
+ if (!kernelmode_fixup_or_oops(regs, error_code, address, 0, 0))
+ return;
}

if (fault & VM_FAULT_OOM) {
/* Kernel mode? Handle exceptions or die: */
- if (!user_mode(regs)) {
- kernelmode_fixup_or_oops(regs, error_code, address,
- SIGSEGV, SEGV_MAPERR);
+ if (!kernelmode_fixup_or_oops(regs, error_code, address, SIGSEGV, SEGV_MAPERR))
return;
- }

/*
* We ran out of memory, call the OOM killer, and return the
--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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