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Message-ID: <1425164559.4645.157.camel@kernel.crashing.org>
Date: Sun, 01 Mar 2015 10:02:39 +1100
From: Benjamin Herrenschmidt <benh@...nel.crashing.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
linux-mm <linux-mm@...ck.org>
Subject: Re: Generic page fault (Was: libsigsegv ....)
On Sun, 2015-03-01 at 09:16 +1100, Benjamin Herrenschmidt wrote:
> So for error handling, I'm trying to simply return the VM_FAULT_* flags
> from generic_page_fault see where that takes us. That's a way to avoid
> passing an arch specific struct around. It also allows my hack to
> account major faults with the hypervisor to be done outside the generic
> code completely (no hook).
>
> .../...
Here's what it looks like for x86 only and without completely sorting
out the fatal signal business. However I might still have to do the
arch pointer you mentioned for sparc and possibly other archs, but so
far it looks better already.
Note that if I add that arch pointer, I might stop messing around
or even returning "fault" and instead just return a simple enum
minor,major,error and let inline arch hooks populate the arch pointer
with the error details in whatever fashion the arch prefers. However
I suspect they'll all end up with sig and si_code in there...
Anyway, here's the current patch:
arch/x86/include/asm/fault.h | 21 ++++
arch/x86/mm/fault.c | 233 ++++---------------------------------------
include/linux/fault.h | 24 +++++
include/linux/mm.h | 5 +-
mm/Makefile | 2 +-
mm/fault.c | 196 ++++++++++++++++++++++++++++++++++++
6 files changed, 266 insertions(+), 215 deletions(-)
create mode 100644 arch/x86/include/asm/fault.h
create mode 100644 include/linux/fault.h
create mode 100644 mm/fault.c
diff --git a/arch/x86/include/asm/fault.h b/arch/x86/include/asm/fault.h
new file mode 100644
index 0000000..04263ec
--- /dev/null
+++ b/arch/x86/include/asm/fault.h
@@ -0,0 +1,21 @@
+#ifndef _ASM_X86_FAULT_H
+#define _ASM_X86_FAULT_H
+
+#include <linux/types.h>
+#include <asm/ptrace.h>
+
+/* Check if the stack is allowed to grow during a user page fault */
+static inline bool stack_can_grow(struct pt_regs *regs, unsigned long flags,
+ unsigned long address,
+ struct vm_area_struct *vma)
+{
+ /*
+ * Accessing the stack below %sp is always a bug.
+ * The large cushion allows instructions like enter
+ * and pusha to work. ("enter $65535, $31" pushes
+ * 32 pointers and then decrements %sp by 65535.)
+ */
+ return address + 65536 + 32 * sizeof(unsigned long) >= regs->sp;
+}
+
+#endif /* _ASM_X86_FAULT_H */
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index ede025f..19a8a91 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -13,6 +13,7 @@
#include <linux/hugetlb.h> /* hstate_index_to_shift */
#include <linux/prefetch.h> /* prefetchw */
#include <linux/context_tracking.h> /* exception_enter(), ... */
+#include <linux/fault.h>
#include <asm/traps.h> /* dotraplinkage, ... */
#include <asm/pgalloc.h> /* pgd_*(), ... */
@@ -748,8 +749,7 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
printk(KERN_CONT "\n");
}
-static void
-__bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
+static void __bad_area(struct pt_regs *regs, unsigned long error_code,
unsigned long address, int si_code)
{
struct task_struct *tsk = current;
@@ -804,39 +804,10 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
no_context(regs, error_code, address, SIGSEGV, si_code);
}
-static noinline void
-bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
- unsigned long address)
-{
- __bad_area_nosemaphore(regs, error_code, address, SEGV_MAPERR);
-}
-
-static void
-__bad_area(struct pt_regs *regs, unsigned long error_code,
- unsigned long address, int si_code)
-{
- struct mm_struct *mm = current->mm;
-
- /*
- * Something tried to access memory that isn't in our memory map..
- * Fix it, but check if it's kernel or user first..
- */
- up_read(&mm->mmap_sem);
-
- __bad_area_nosemaphore(regs, error_code, address, si_code);
-}
-
-static noinline void
-bad_area(struct pt_regs *regs, unsigned long error_code, unsigned long address)
-{
- __bad_area(regs, error_code, address, SEGV_MAPERR);
-}
-
-static noinline void
-bad_area_access_error(struct pt_regs *regs, unsigned long error_code,
- unsigned long address)
+static inline void bad_area(struct pt_regs *regs, unsigned long error_code,
+ unsigned long address, int si_code)
{
- __bad_area(regs, error_code, address, SEGV_ACCERR);
+ __bad_area(regs, error_code, address, si_code);
}
static void
@@ -871,40 +842,6 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
force_sig_info_fault(SIGBUS, code, address, tsk, fault);
}
-static noinline void
-mm_fault_error(struct pt_regs *regs, unsigned long error_code,
- unsigned long address, unsigned int fault)
-{
- if (fatal_signal_pending(current) && !(error_code & PF_USER)) {
- no_context(regs, error_code, address, 0, 0);
- return;
- }
-
- if (fault & VM_FAULT_OOM) {
- /* Kernel mode? Handle exceptions or die: */
- if (!(error_code & PF_USER)) {
- no_context(regs, error_code, address,
- SIGSEGV, SEGV_MAPERR);
- return;
- }
-
- /*
- * We ran out of memory, call the OOM killer, and return the
- * userspace (which will retry the fault, or kill us if we got
- * oom-killed):
- */
- pagefault_out_of_memory();
- } else {
- if (fault & (VM_FAULT_SIGBUS|VM_FAULT_HWPOISON|
- VM_FAULT_HWPOISON_LARGE))
- do_sigbus(regs, error_code, address, fault);
- else if (fault & VM_FAULT_SIGSEGV)
- bad_area_nosemaphore(regs, error_code, address);
- else
- BUG();
- }
-}
-
static int spurious_fault_check(unsigned long error_code, pte_t *pte)
{
if ((error_code & PF_WRITE) && !pte_write(*pte))
@@ -998,27 +935,6 @@ NOKPROBE_SYMBOL(spurious_fault);
int show_unhandled_signals = 1;
-static inline int
-access_error(unsigned long error_code, struct vm_area_struct *vma)
-{
- if (error_code & PF_WRITE) {
- /* write, present and write, not present: */
- if (unlikely(!(vma->vm_flags & VM_WRITE)))
- return 1;
- return 0;
- }
-
- /* read, present: */
- if (unlikely(error_code & PF_PROT))
- return 1;
-
- /* read, not present: */
- if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))))
- return 1;
-
- return 0;
-}
-
static int fault_in_kernel_space(unsigned long address)
{
return address >= TASK_SIZE_MAX;
@@ -1054,11 +970,10 @@ static noinline void
__do_page_fault(struct pt_regs *regs, unsigned long error_code,
unsigned long address)
{
- struct vm_area_struct *vma;
struct task_struct *tsk;
struct mm_struct *mm;
- int fault, major = 0;
unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+ unsigned int fault;
tsk = current;
mm = tsk->mm;
@@ -1107,7 +1022,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
* Don't take the mm semaphore here. If we fixup a prefetch
* fault we could otherwise deadlock:
*/
- bad_area_nosemaphore(regs, error_code, address);
+ bad_area(regs, error_code, address, SEGV_MAPERR);
return;
}
@@ -1120,7 +1035,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
pgtable_bad(regs, error_code, address);
if (unlikely(smap_violation(error_code, regs))) {
- bad_area_nosemaphore(regs, error_code, address);
+ bad_area(regs, error_code, address, SEGV_MAPERR);
return;
}
@@ -1129,13 +1044,14 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
* in an atomic region then we must not take the fault:
*/
if (unlikely(in_atomic() || !mm)) {
- bad_area_nosemaphore(regs, error_code, address);
+ bad_area(regs, error_code, address, SEGV_MAPERR);
return;
}
/*
* It's safe to allow irq's after cr2 has been saved and the
* vmalloc fault has been handled.
+
*
* User-mode registers count as a user access even for any
* potential system fault or CPU buglet:
@@ -1143,138 +1059,29 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
if (user_mode_vm(regs)) {
local_irq_enable();
error_code |= PF_USER;
- flags |= FAULT_FLAG_USER;
} else {
if (regs->flags & X86_EFLAGS_IF)
local_irq_enable();
}
- perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
-
+ if (error_code & PF_USER)
+ flags |= FAULT_FLAG_USER;
if (error_code & PF_WRITE)
flags |= FAULT_FLAG_WRITE;
+ if (error_code & PF_PROT)
+ flags |= FAULT_FLAG_PROT;
- /*
- * When running in the kernel we expect faults to occur only to
- * addresses in user space. All other faults represent errors in
- * the kernel and should generate an OOPS. Unfortunately, in the
- * case of an erroneous fault occurring in a code path which already
- * holds mmap_sem we will deadlock attempting to validate the fault
- * against the address space. Luckily the kernel only validly
- * references user space from well defined areas of code, which are
- * listed in the exceptions table.
- *
- * As the vast majority of faults will be valid we will only perform
- * the source reference check when there is a possibility of a
- * deadlock. Attempt to lock the address space, if we cannot we then
- * validate the source. If this is invalid we can skip the address
- * space check, thus avoiding the deadlock:
- */
- if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
- if ((error_code & PF_USER) == 0 &&
- !search_exception_tables(regs->ip)) {
- bad_area_nosemaphore(regs, error_code, address);
- return;
- }
-retry:
- down_read(&mm->mmap_sem);
- } else {
- /*
- * The above down_read_trylock() might have succeeded in
- * which case we'll have missed the might_sleep() from
- * down_read():
- */
- might_sleep();
- }
-
- vma = find_vma(mm, address);
- if (unlikely(!vma)) {
- bad_area(regs, error_code, address);
- return;
- }
- if (likely(vma->vm_start <= address))
- goto good_area;
- if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) {
- bad_area(regs, error_code, address);
+ fault = generic_page_fault(regs, tsk, flags, address);
+ if (unlikely(fault & VM_FAULT_SIGSEGV)) {
+ bad_area(regs, error_code, address,
+ (fault & VM_FAULT_ACCESS) ? SEGV_MAPERR : SEGV_ACCERR);
return;
}
- if (error_code & PF_USER) {
- /*
- * Accessing the stack below %sp is always a bug.
- * The large cushion allows instructions like enter
- * and pusha to work. ("enter $65535, $31" pushes
- * 32 pointers and then decrements %sp by 65535.)
- */
- if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
- bad_area(regs, error_code, address);
- return;
- }
- }
- if (unlikely(expand_stack(vma, address))) {
- bad_area(regs, error_code, address);
+ if (unlikely(fault & VM_FAULT_SIGBUS)) {
+ do_sigbus(regs, error_code, address, fault);
return;
}
- /*
- * Ok, we have a good vm_area for this memory access, so
- * we can handle it..
- */
-good_area:
- if (unlikely(access_error(error_code, vma))) {
- bad_area_access_error(regs, error_code, address);
- return;
- }
-
- /*
- * If for any reason at all we couldn't handle the fault,
- * make sure we exit gracefully rather than endlessly redo
- * the fault. Since we never set FAULT_FLAG_RETRY_NOWAIT, if
- * we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.
- */
- fault = handle_mm_fault(mm, vma, address, flags);
- major |= fault & VM_FAULT_MAJOR;
-
- /*
- * If we need to retry the mmap_sem has already been released,
- * and if there is a fatal signal pending there is no guarantee
- * that we made any progress. Handle this case first.
- */
- if (unlikely(fault & VM_FAULT_RETRY)) {
- /* Retry at most once */
- if (flags & FAULT_FLAG_ALLOW_RETRY) {
- flags &= ~FAULT_FLAG_ALLOW_RETRY;
- flags |= FAULT_FLAG_TRIED;
- if (!fatal_signal_pending(tsk))
- goto retry;
- }
-
- /* User mode? Just return to handle the fatal exception */
- if (flags & FAULT_FLAG_USER)
- return;
-
- /* Not returning to user mode? Handle exceptions or die: */
- no_context(regs, error_code, address, SIGBUS, BUS_ADRERR);
- return;
- }
-
- up_read(&mm->mmap_sem);
- if (unlikely(fault & VM_FAULT_ERROR)) {
- mm_fault_error(regs, error_code, address, fault);
- return;
- }
-
- /*
- * Major/minor page fault accounting. If any of the events
- * returned VM_FAULT_MAJOR, we account it as a major fault.
- */
- if (major) {
- tsk->maj_flt++;
- perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, regs, address);
- } else {
- tsk->min_flt++;
- perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, address);
- }
-
check_v8086_mode(regs, address, tsk);
}
NOKPROBE_SYMBOL(__do_page_fault);
diff --git a/include/linux/fault.h b/include/linux/fault.h
new file mode 100644
index 0000000..11c567e
--- /dev/null
+++ b/include/linux/fault.h
@@ -0,0 +1,24 @@
+#ifndef __FAULT_H
+#define __FAULT_H
+
+/* Generic page fault stuff */
+
+#include <asm/fault.h>
+
+/* Returns the fault flags from handle_mm_fault() with the addition
+ * that:
+ *
+ * - On an error, either VM_FAULT_SIGSEGV or VM_FAULT_SIGBUS will
+ * always be set in addition to other flags
+ *
+ * - VM_FAULT_ACCESS will be added to VM_FAULT_SIGSEGV for access
+ * protection faults
+ *
+ * - VM_FAULT_MAJOR will be set even if there was a retry
+ *
+ * - XXX FIXME: VM_FAULT_SIGBUS will be set on a fatal signal
+ */
+unsigned int generic_page_fault(struct pt_regs *regs, struct task_struct *tsk,
+ unsigned long flags, unsigned long address);
+
+#endif /* __FAULT_H */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 47a9392..5578eba 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -211,6 +211,9 @@ extern pgprot_t protection_map[16];
#define FAULT_FLAG_KILLABLE 0x10 /* The fault task is in SIGKILL killable region */
#define FAULT_FLAG_TRIED 0x20 /* Second try */
#define FAULT_FLAG_USER 0x40 /* The fault originated in userspace */
+#define FAULT_FLAG_EXEC 0x80 /* The fault was an instruction fetch */
+#define FAULT_FLAG_PROT 0x100 /* HW detected protection fault */
+#define FAULT_FLAG_NO_STK_GROW 0x200 /* Fault is not allowed to grow stack */
/*
* vm_fault is filled by the the pagefault handler and passed to the vma's
@@ -1098,7 +1101,7 @@ static inline int page_mapped(struct page *page)
#define VM_FAULT_HWPOISON 0x0010 /* Hit poisoned small page */
#define VM_FAULT_HWPOISON_LARGE 0x0020 /* Hit poisoned large page. Index encoded in upper bits */
#define VM_FAULT_SIGSEGV 0x0040
-
+#define VM_FAULT_ACCESS 0x0080 /* in addition to VM_FAULT_SIGSEGV */
#define VM_FAULT_NOPAGE 0x0100 /* ->fault installed the pte, not return page */
#define VM_FAULT_LOCKED 0x0200 /* ->fault locked the returned page */
#define VM_FAULT_RETRY 0x0400 /* ->fault blocked, must retry */
diff --git a/mm/Makefile b/mm/Makefile
index 3c1caa2..f647ff1 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -8,7 +8,7 @@ KASAN_SANITIZE_slub.o := n
mmu-y := nommu.o
mmu-$(CONFIG_MMU) := gup.o highmem.o memory.o mincore.o \
mlock.o mmap.o mprotect.o mremap.o msync.o rmap.o \
- vmalloc.o pagewalk.o pgtable-generic.o
+ vmalloc.o pagewalk.o pgtable-generic.o fault.o
ifdef CONFIG_CROSS_MEMORY_ATTACH
mmu-$(CONFIG_MMU) += process_vm_access.o
diff --git a/mm/fault.c b/mm/fault.c
new file mode 100644
index 0000000..3bf4583
--- /dev/null
+++ b/mm/fault.c
@@ -0,0 +1,196 @@
+#include <linux/mm.h>
+#include <linux/sched.h>
+#include <linux/perf_event.h>
+#include <linux/module.h>
+
+#include <asm/fault.h>
+
+static noinline unsigned int mm_fault_error(struct pt_regs *regs,
+ unsigned int flags,
+ unsigned long address,
+ unsigned int fault)
+{
+ /* XXX Hack in VM_FAULT_SIGBUS, we need to fix that */
+ if (fatal_signal_pending(current) && !(flags & FAULT_FLAG_USER))
+ return fault | VM_FAULT_SIGBUS;
+
+ if (fault & VM_FAULT_OOM) {
+ /* Kernel mode? Handle exceptions or die: */
+ if (!(flags & FAULT_FLAG_USER))
+ return fault | VM_FAULT_SIGSEGV;
+
+ /*
+ * We ran out of memory, call the OOM killer, and return the
+ * userspace (which will retry the fault, or kill us if we got
+ * oom-killed):
+ */
+ pagefault_out_of_memory();
+ } else {
+ if (fault & (VM_FAULT_SIGBUS|VM_FAULT_HWPOISON|
+ VM_FAULT_HWPOISON_LARGE))
+ return fault | VM_FAULT_SIGBUS;
+ else if (fault & VM_FAULT_SIGSEGV)
+ return fault;
+ else
+ BUG();
+ }
+
+ /* Clear error conditions */
+ return fault & ~VM_FAULT_ERROR;
+}
+
+/* Access validity check */
+static inline bool access_error(struct pt_regs *regs, unsigned long flags,
+ struct vm_area_struct *vma)
+{
+ /* Write fault to a non-writeable VMA */
+ if (flags & FAULT_FLAG_WRITE) {
+ if (unlikely(!(vma->vm_flags & VM_WRITE)))
+ return true;
+ return false;
+ }
+
+ /* Exec fault to a non-executable VMA */
+ if (flags & FAULT_FLAG_EXEC) {
+ if (unlikely(!(vma->vm_flags & VM_WRITE)))
+ return true;
+ return false;
+ }
+
+ /* Other HW detected protection fault */
+ if (unlikely(flags & PF_PROT))
+ return true;
+
+ /* No access allowed to that VMA */
+ if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))))
+ return true;
+
+ return false;
+}
+
+unsigned int generic_page_fault(struct pt_regs *regs, struct task_struct *tsk,
+ unsigned int flags, unsigned long address)
+{
+ struct vm_area_struct *vma;
+ struct mm_struct *mm;
+ int fault, major = 0;
+
+ mm = tsk->mm;
+
+ perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
+
+ /*
+ * When running in the kernel we expect faults to occur only to
+ * addresses in user space. All other faults represent errors in
+ * the kernel and should generate an OOPS. Unfortunately, in the
+ * case of an erroneous fault occurring in a code path which already
+ * holds mmap_sem we will deadlock attempting to validate the fault
+ * against the address space. Luckily the kernel only validly
+ * references user space from well defined areas of code, which are
+ * listed in the exceptions table.
+ *
+ * As the vast majority of faults will be valid we will only perform
+ * the source reference check when there is a possibility of a
+ * deadlock. Attempt to lock the address space, if we cannot we then
+ * validate the source. If this is invalid we can skip the address
+ * space check, thus avoiding the deadlock:
+ */
+ if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
+ if (!(flags & FAULT_FLAG_USER) &&
+ !search_exception_tables(GET_IP(regs))) {
+ return VM_FAULT_SIGSEGV;
+ }
+retry:
+ down_read(&mm->mmap_sem);
+ } else {
+ /*
+ * The above down_read_trylock() might have succeeded in
+ * which case we'll have missed the might_sleep() from
+ * down_read():
+ */
+ might_sleep();
+ }
+
+ vma = find_vma(mm, address);
+ if (unlikely(!vma))
+ goto bad_area;
+ if (likely(vma->vm_start <= address))
+ goto good_area;
+ if (unlikely(!(vma->vm_flags & VM_GROWSDOWN)))
+ goto bad_area;
+ if (unlikely((flags & FAULT_FLAG_USER) &&
+ !stack_can_grow(regs, flags, address, vma)))
+ goto bad_area;
+ if (unlikely(expand_stack(vma, address)))
+ goto bad_area;
+
+ /*
+ * Ok, we have a good vm_area for this memory access, so
+ * we can handle it..
+ */
+good_area:
+ if (unlikely(access_error(regs, flags, vma)))
+ goto bad_access;
+
+ /*
+ * If for any reason at all we couldn't handle the fault,
+ * make sure we exit gracefully rather than endlessly redo
+ * the fault. Since we never set FAULT_FLAG_RETRY_NOWAIT, if
+ * we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.
+ */
+ fault = handle_mm_fault(mm, vma, address, flags);
+ major |= fault & VM_FAULT_MAJOR;
+
+ /*
+ * If we need to retry the mmap_sem has already been released,
+ * and if there is a fatal signal pending there is no guarantee
+ * that we made any progress. Handle this case first.
+ */
+ if (unlikely(fault & VM_FAULT_RETRY)) {
+ /* Retry at most once */
+ if (flags & FAULT_FLAG_ALLOW_RETRY) {
+ flags &= ~FAULT_FLAG_ALLOW_RETRY;
+ flags |= FAULT_FLAG_TRIED;
+ if (!fatal_signal_pending(tsk))
+ goto retry;
+ }
+
+ /* User mode? Just return to handle the fatal exception */
+ if (flags & FAULT_FLAG_USER)
+ return FAULT_NO_ERR;
+
+ /* Not returning to user mode? Handle exceptions or die: */
+ /* XXX mimmic x86, but might not be best */
+ return fault | VM_FAULT_SIGBUS;
+ }
+
+ up_read(&mm->mmap_sem);
+ if (unlikely(fault & VM_FAULT_ERROR))
+ return mm_fault_error(regs, flags, address, fault);
+
+ /*
+ * Major/minor page fault accounting. If any of the events
+ * returned VM_FAULT_MAJOR, we account it as a major fault.
+ */
+ if (major) {
+ tsk->maj_flt++;
+ perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, regs, address);
+
+ /* Restore the major flag, it might have been lost in case of
+ * retry and the arch might care
+ */
+ if (major)
+ fault |= VM_FAULT_MAJOR;
+ } else {
+ tsk->min_flt++;
+ perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, address);
+ }
+ return fault;
+
+ bad_area:
+ up_read(&mm->mmap_sem);
+ return VM_FAULT_SIGSEGV;
+ bad_access:
+ up_read(&mm->mmap_sem);
+ return VM_FAULT_SIGSEGV | VM_FAULT_ACCESS;
+}
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