Re: [x86] BUG: unable to handle kernel paging request at 00740060

From: Peter Zijlstra
Date: Wed Oct 09 2013 - 10:07:57 EST

On Wed, Oct 09, 2013 at 02:43:10PM +0200, Oleg Nesterov wrote:
> I'm afraid I am wrong, my asm skills are close to zero... but this
> code looks wrong to me, and this can explain the oopses.
>
> > task_work_add:
> > pushl %ebp #
> > movl %esp, %ebp #,
> > pushl %edi #
> > pushl %esi #
> > pushl %ebx #
> > subl $12, %esp #,
> > call mcount
> > movl %eax, %edi # task, task
> > movl %edx, -16(%ebp) # work, %sfp
> > movb %cl, -21(%ebp) # notify, %sfp
> > .p2align 4,,15
> > .L3:
> > movl 904(%edi), %esi # task_3(D)->task_works, head
> > cmpl $work_exited, %esi #, head
> > sete %bl #, D.14145
> > andl $255, %ebx #, D.14145
> > xorl %ecx, %ecx #
> > movl %ebx, %edx # D.14145,
> > movl $______f.14042, %eax #,
> > call ftrace_likely_update #
> > testl %ebx, %ebx # D.14145
> > jne .L4 #,
> > movl -16(%ebp), %edx # %sfp,
> > movl %esi, (%edx) # head, work_13(D)->next
> > movl %esi, %eax # head, __ret
> > #APP
> > # 34 "/c/wfg/tip/kernel/task_work.c" 1
> > cmpxchgl %edx,904(%edi) #, *__ptr_16
> > # 0 "" 2
> > #NO_APP
> > cmpl %eax, %esi # __ret, head
> > jne .L3 #,
>
> OK, we added the new work successfully, we should return 0. If we return
> non-zero, fput() (the likely caller) assumes that it should use the workqueues
> to close/free this file. Then later task_work_run() will do __fput() again.
>
> > cmpb $0, -21(%ebp) #, %sfp
> > je .L5 #,
> > movl 4(%edi), %eax # task_3(D)->stack, task_3(D)->stack
> > #APP
> > # 208 "/c/wfg/tip/arch/x86/include/asm/bitops.h" 1
> > bts $1, 8(%eax); jc .L2 #, MEM[(volatile long unsigned int *)D.14203_29],
>
> This is set_notify_resume(). Probably !CONFIG_SMP (I do not see kick_process).
>
> > # 0 "" 2
> > #NO_APP
> > .L5:
> > movl $0, -20(%ebp) #, %sfp
> > .L2:
> > movl -20(%ebp), %eax # %sfp,
>
> This is what we are going to return. But note that -20(%ebp) was not
> initialized if TIF_NOTIFY_RESUME was already set, "jc .L2" skips .L5
> above. IOW, in this case we seem to return a random value from stack.

I think you're quite right, and I can confirm I can reproduce this with
gcc-4.8.1 and Wu's .config:

.p2align 4,,15
.globl task_work_add
.type task_work_add, @function
task_work_add:
pushl %ebp #
movl %esp, %ebp #,
pushl %edi #
pushl %esi #
pushl %ebx #
subl $12, %esp #,
call mcount
movl %eax, %esi # task, task
movl %edx, %edi # work, work
movl %ecx, -24(%ebp) # notify, %sfp
jmp .L4 #
.p2align 4,,15
.L9:
movl %ebx, (%edi) # __old, work_15(D)->next
movl %ebx, %eax # __old, __ret
#APP
# 34 "/usr/src/linux-2.6/kernel/task_work.c" 1
cmpxchgl %edi,904(%esi) # work, *__ptr_17
# 0 "" 2
#NO_APP
cmpl %eax, %ebx # __ret, __old
je .L8 #,
.L4:
movl 904(%esi), %ebx # task_7(D)->task_works, __old
cmpl $work_exited, %ebx #, __old
sete -13(%ebp) #, %sfp
xorl %edx, %edx # ______r
movb -13(%ebp), %dl # %sfp, ______r
xorl %ecx, %ecx #
movl $______f.14204, %eax #,
call ftrace_likely_update #
cmpb $0, -13(%ebp) #, %sfp
je .L9 #,
movl $-3, -20(%ebp) #, %sfp
.L2:
movl -20(%ebp), %eax # %sfp,
addl $12, %esp #,
popl %ebx #
popl %esi #
popl %edi #
popl %ebp #
ret
.p2align 4,,15
.L8:
cmpb $0, -24(%ebp) #, %sfp
je .L6 #,
movl 4(%esi), %eax # task_7(D)->stack, task_7(D)->stack
#APP
# 208 "/usr/src/linux-2.6/arch/x86/include/asm/bitops.h" 1
bts $1, 8(%eax); jc .L2 #, MEM[(volatile long unsigned int *)_23],
# 0 "" 2
#NO_APP
.L6:
movl $0, -20(%ebp) #, %sfp
movl -20(%ebp), %eax # %sfp,
addl $12, %esp #,
popl %ebx #
popl %esi #
popl %edi #
popl %ebp #
ret
.size task_work_add, .-task_work_add

Once I force a x86_64 build using the 'same' config it goes away and
generates 'sensible' code again (although I don't see why L9 isn't
merged with L2):

.p2align 4,,15
.globl task_work_add
.type task_work_add, @function
task_work_add:
call __fentry__
pushq %rbp #
movq %rsp, %rbp #,
pushq %r15 #
pushq %r14 #
movl %edx, %r14d # notify, notify
pushq %r13 #
movq %rsi, %r13 # work, work
pushq %r12 #
movq %rdi, %r12 # task, task
pushq %rbx #
jmp .L4 #
.p2align 4,,10
.p2align 3
.L11:
movq %rbx, 0(%r13) # __old, work_17(D)->next
movq %rbx, %rax # __old, __ret
#APP
# 34 "/usr/src/linux-2.6/kernel/task_work.c" 1
cmpxchgq %r13,1312(%r12) # work, *__ptr_19
# 0 "" 2
#NO_APP
cmpq %rax, %rbx # __ret, __old
je .L10 #,
.L4:
movq 1312(%r12), %rbx # task_7(D)->task_works, __old
movq $______f.14855, %rdi #,
cmpq $work_exited, %rbx #, __old
sete %r15b #, tmp72
xorl %edx, %edx #
movzbl %r15b, %esi # tmp72, ______r
call ftrace_likely_update #
testb %r15b, %r15b # tmp72
je .L11 #,
movl $-3, %eax #, D.15034
.L2:
popq %rbx #
popq %r12 #
popq %r13 #
popq %r14 #
popq %r15 #
popq %rbp #
ret
.p2align 4,,10
.p2align 3
.L10:
xorl %eax, %eax # D.15034
testb %r14b, %r14b # notify
je .L2 #,
movq 8(%r12), %rdx # task_7(D)->stack, task_7(D)->stack
#APP
# 208 "/usr/src/linux-2.6/arch/x86/include/asm/bitops.h" 1
bts $1, 16(%rdx); jc .L9 #, MEM[(volatile long unsigned int *)_25],
# 0 "" 2
#NO_APP
.L9:
popq %rbx #
popq %r12 #
popq %r13 #
popq %r14 #
popq %r15 #
popq %rbp #
ret
.size task_work_add, .-task_work_add


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