Andreas reported that commit ee0a49a687 ("powerpc/uaccess: Switch
__put_user_size_allowed() to __put_user_asm_goto()") broke
CLONE_CHILD_SETTID.
Further inspection showed that the put_user() in schedule_tail() was
missing entirely, the store not emitted by the compiler.
<.schedule_tail>:
mflr r0
std r0,16(r1)
stdu r1,-112(r1)
bl <.finish_task_switch>
ld r9,2496(r3)
cmpdi cr7,r9,0
bne cr7,<.schedule_tail+0x60>
ld r3,392(r13)
ld r9,1392(r3)
cmpdi cr7,r9,0
beq cr7,<.schedule_tail+0x3c>
li r4,0
li r5,0
bl <.__task_pid_nr_ns>
nop
bl <.calculate_sigpending>
nop
addi r1,r1,112
ld r0,16(r1)
mtlr r0
blr
nop
nop
nop
bl <.__balance_callback>
b <.schedule_tail+0x1c>
Notice there are no stores other than to the stack. There should be a
stw in there for the store to current->set_child_tid.
This is only seen with GCC 4.9 era compilers (tested with 4.9.3 and
4.9.4), and only when CONFIG_PPC_KUAP is disabled.
When CONFIG_PPC_KUAP=y, the inline asm that's part of the isync()
and mtspr() inlined via allow_user_access() seems to be enough to
avoid the bug.
We already have a macro to work around this (or a similar bug), called
asm_volatile_goto which includes an empty asm block to tickle the
compiler into generating the right code. So use that.
With this applied the code generation looks more like it will work:
<.schedule_tail>:
mflr r0
std r31,-8(r1)
std r0,16(r1)
stdu r1,-144(r1)
std r3,112(r1)
bl <._mcount>
nop
ld r3,112(r1)
bl <.finish_task_switch>
ld r9,2624(r3)
cmpdi cr7,r9,0
bne cr7,<.schedule_tail+0xa0>
ld r3,2408(r13)
ld r31,1856(r3)
cmpdi cr7,r31,0
beq cr7,<.schedule_tail+0x80>
li r4,0
li r5,0
bl <.__task_pid_nr_ns>
nop
li r9,-1
clrldi r9,r9,12
cmpld cr7,r31,r9
bgt cr7,<.schedule_tail+0x80>
lis r9,16
rldicr r9,r9,32,31
subf r9,r31,r9
cmpldi cr7,r9,3
ble cr7,<.schedule_tail+0x80>
li r9,0
stw r3,0(r31) <-- stw
nop
bl <.calculate_sigpending>
nop
addi r1,r1,144
ld r0,16(r1)
ld r31,-8(r1)
mtlr r0
blr
nop
bl <.__balance_callback>
b <.schedule_tail+0x30>
Fixes: ee0a49a687 ("powerpc/uaccess: Switch __put_user_size_allowed() to __put_user_asm_goto()")
Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Tested-by: Andreas Schwab <schwab@linux-m68k.org>
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201104111742.672142-1-mpe@ellerman.id.au
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