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SubjectREAD_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
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[ trimmed CC a bit ]

Peter Zijlstra <peterz@infradead.org> writes:
> On Fri, Dec 06, 2019 at 11:46:11PM +1100, Michael Ellerman wrote:
...
> you write:
>
> "Currently bitops-instrumented.h assumes that the architecture provides
> atomic, non-atomic and locking bitops (e.g. both set_bit and __set_bit).
> This is true on x86 and s390, but is not always true: there is a
> generic bitops/non-atomic.h header that provides generic non-atomic
> operations, and also a generic bitops/lock.h for locking operations."
>
> Is there any actual benefit for PPC to using their own atomic bitops
> over bitops/lock.h ? I'm thinking that the generic code is fairly
> optimal for most LL/SC architectures.

Yes and no :)

Some of the generic versions don't generate good code compared to our
versions, but that's because READ_ONCE() is triggering stack protector
to be enabled.

For example, comparing an out-of-line copy of the generic and ppc
versions of test_and_set_bit_lock():

1 <generic_test_and_set_bit_lock>: 1 <ppc_test_and_set_bit_lock>:
2 addis r2,r12,361
3 addi r2,r2,-4240
4 stdu r1,-48(r1)
5 rlwinm r8,r3,29,3,28
6 clrlwi r10,r3,26 2 rldicl r10,r3,58,6
7 ld r9,3320(r13)
8 std r9,40(r1)
9 li r9,0
10 li r9,1 3 li r9,1
4 clrlwi r3,r3,26
5 rldicr r10,r10,3,60
11 sld r9,r9,r10 6 sld r3,r9,r3
12 add r10,r4,r8 7 add r4,r4,r10
13 ldx r8,r4,r8
14 and. r8,r9,r8
15 bne 34f
16 ldarx r7,0,r10 8 ldarx r9,0,r4,1
17 or r8,r9,r7 9 or r10,r9,r3
18 stdcx. r8,0,r10 10 stdcx. r10,0,r4
19 bne- 16b 11 bne- 8b
20 isync 12 isync
21 and r9,r7,r9 13 and r3,r3,r9
22 addic r7,r9,-1 14 addic r9,r3,-1
23 subfe r7,r7,r9 15 subfe r3,r9,r3
24 ld r9,40(r1)
25 ld r10,3320(r13)
26 xor. r9,r9,r10
27 li r10,0
28 mr r3,r7
29 bne 36f
30 addi r1,r1,48
31 blr 16 blr
32 nop
33 nop
34 li r7,1
35 b 24b
36 mflr r0
37 std r0,64(r1)
38 bl <__stack_chk_fail+0x8>


If you squint, the generated code for the actual logic is pretty similar, but
the stack protector gunk makes a big mess. It's particularly bad here
because the ppc version doesn't even need a stack frame.

I've also confirmed that even when test_and_set_bit_lock() is inlined
into an actual call site the stack protector logic still triggers.

eg, if I make two versions of ext4_resize_begin() which call the generic or ppc
version of test_and_set_bit_lock(), the generic version gets a bunch of extra
stack protector code.

1 c0000000005336e0 <ext4_resize_begin_generic>: 1 c0000000005335b0 <ext4_resize_begin_ppc>:
2 addis r2,r12,281 2 addis r2,r12,281
3 addi r2,r2,-12256 3 addi r2,r2,-11952
4 mflr r0 4 mflr r0
5 bl <_mcount> 5 bl <_mcount>
6 mflr r0 6 mflr r0
7 std r31,-8(r1) 7 std r31,-8(r1)
8 std r30,-16(r1) 8 std r30,-16(r1)
9 mr r31,r3 9 mr r31,r3
10 li r3,24 10 li r3,24
11 std r0,16(r1) 11 std r0,16(r1)
12 stdu r1,-128(r1) 12 stdu r1,-112(r1)
13 ld r9,3320(r13)
14 std r9,104(r1)
15 li r9,0
16 ld r30,920(r31) 13 ld r30,920(r31)
17 bl <capable+0x8> 14 bl <capable+0x8>
18 nop 15 nop
19 cmpdi cr7,r3,0 16 cmpdi cr7,r3,0
20 beq cr7,<ext4_resize_begin_generic+0xf0> 17 beq cr7,<ext4_resize_begin_ppc+0xc0>
21 ld r9,920(r31) 18 ld r9,920(r31)
22 ld r10,96(r30) 19 ld r10,96(r30)
23 lwz r7,84(r30) 20 lwz r7,84(r30)
24 ld r8,104(r9) 21 ld r8,104(r9)
25 ld r10,24(r10) 22 ld r10,24(r10)
26 lwz r8,20(r8) 23 lwz r8,20(r8)
27 srd r10,r10,r7 24 srd r10,r10,r7
28 cmpd cr7,r10,r8 25 cmpd cr7,r10,r8
29 bne cr7,<ext4_resize_begin_generic+0x128> 26 bne cr7,<ext4_resize_begin_ppc+0xf8>
30 lhz r10,160(r9) 27 lhz r10,160(r9)
31 andi. r10,r10,2 28 andi. r10,r10,2
32 bne <ext4_resize_begin_generic+0x100>
33 ld r10,560(r9)
34 andi. r10,r10,1
35 bne <ext4_resize_begin_generic+0xe0> 29 bne <ext4_resize_begin_ppc+0xd0>
36 addi r7,r9,560 30 addi r9,r9,560
37 li r8,1 31 li r10,1
38 ldarx r10,0,r7 32 ldarx r3,0,r9,1
39 or r6,r8,r10 33 or r8,r3,r10
40 stdcx. r6,0,r7 34 stdcx. r8,0,r9
41 bne- <ext4_resize_begin_generic+0x90> 35 bne- <ext4_resize_begin_ppc+0x78>
42 isync 36 isync
37 clrldi r3,r3,63
43 andi. r9,r10,1 38 addi r3,r3,-1
44 li r3,0 39 rlwinm r3,r3,0,27,27
45 bne <ext4_resize_begin_generic+0xe0> 40 addi r3,r3,-16
46 ld r9,104(r1)
47 ld r10,3320(r13)
48 xor. r9,r9,r10
49 li r10,0
50 bne <ext4_resize_begin_generic+0x158>
51 addi r1,r1,128 41 addi r1,r1,112
52 ld r0,16(r1) 42 ld r0,16(r1)
53 ld r30,-16(r1) 43 ld r30,-16(r1)
54 ld r31,-8(r1) 44 ld r31,-8(r1)
55 mtlr r0 45 mtlr r0
56 blr 46 blr
57 nop 47 nop
58 li r3,-16
59 b <ext4_resize_begin_generic+0xb0>
60 nop 48 nop
61 nop 49 nop
62 li r3,-1 50 li r3,-1
63 b <ext4_resize_begin_generic+0xb0> 51 b <ext4_resize_begin_ppc+0x9c>
64 nop 52 nop
65 nop 53 nop
66 addis r6,r2,-118 54 addis r6,r2,-118
67 addis r4,r2,-140 55 addis r4,r2,-140
68 mr r3,r31 56 mr r3,r31
69 li r5,97 57 li r5,46
70 addi r6,r6,30288 58 addi r6,r6,30288
71 addi r4,r4,3064 59 addi r4,r4,3040
72 bl <__ext4_warning+0x8> 60 bl <__ext4_warning+0x8>
73 nop 61 nop
74 li r3,-1 62 li r3,-1
75 b <ext4_resize_begin_generic+0xb0> 63 b <ext4_resize_begin_ppc+0x9c>
76 ld r9,96(r9) 64 ld r9,96(r9)
77 addis r6,r2,-118 65 addis r6,r2,-118
78 addis r4,r2,-140 66 addis r4,r2,-140
79 mr r3,r31 67 mr r3,r31
80 li r5,87 68 li r5,36
81 addi r6,r6,30240 69 addi r6,r6,30240
82 addi r4,r4,3064 70 addi r4,r4,3040
83 ld r7,24(r9) 71 ld r7,24(r9)
84 bl <__ext4_warning+0x8> 72 bl <__ext4_warning+0x8>
85 nop 73 nop
86 li r3,-1 74 li r3,-1
87 b <ext4_resize_begin_generic+0xb0> 75 b <ext4_resize_begin_ppc+0x9c>
88 bl <__stack_chk_fail+0x8>


If I change the READ_ONCE() in test_and_set_bit_lock():

if (READ_ONCE(*p) & mask)
return 1;

to a regular pointer access:

if (*p & mask)
return 1;

Then the generated code looks more or less the same, except for the extra early
return in the generic version of test_and_set_bit_lock(), and different handling
of the return code by the compiler.

1 <ext4_resize_begin_generic>: 1 <ext4_resize_begin_ppc>:
2 addis r2,r12,281 2 addis r2,r12,281
3 addi r2,r2,-12256 3 addi r2,r2,-11952
4 mflr r0 4 mflr r0
5 bl <_mcount> 5 bl <_mcount>
6 mflr r0 6 mflr r0
7 std r31,-8(r1) 7 std r31,-8(r1)
8 std r30,-16(r1) 8 std r30,-16(r1)
9 mr r31,r3 9 mr r31,r3
10 li r3,24 10 li r3,24
11 std r0,16(r1) 11 std r0,16(r1)
12 stdu r1,-112(r1) 12 stdu r1,-112(r1)
13 ld r30,920(r31) 13 ld r30,920(r31)
14 bl <capable+0x8> 14 bl <capable+0x8>
15 nop 15 nop
16 cmpdi cr7,r3,0 16 cmpdi cr7,r3,0
17 beq cr7,<ext4_resize_begin_generic+0xe0> 17 beq cr7,<ext4_resize_begin_ppc+0xc0>
18 ld r9,920(r31) 18 ld r9,920(r31)
19 ld r10,96(r30) 19 ld r10,96(r30)
20 lwz r7,84(r30) 20 lwz r7,84(r30)
21 ld r8,104(r9) 21 ld r8,104(r9)
22 ld r10,24(r10) 22 ld r10,24(r10)
23 lwz r8,20(r8) 23 lwz r8,20(r8)
24 srd r10,r10,r7 24 srd r10,r10,r7
25 cmpd cr7,r10,r8 25 cmpd cr7,r10,r8
26 bne cr7,<ext4_resize_begin_generic+0x118> 26 bne cr7,<ext4_resize_begin_ppc+0xf8>
27 lhz r10,160(r9) 27 lhz r10,160(r9)
28 andi. r10,r10,2 28 andi. r10,r10,2
29 bne <ext4_resize_begin_generic+0xf0> 29 bne <ext4_resize_begin_ppc+0xd0>
30 ld r10,560(r9)
31 andi. r10,r10,1
32 bne <ext4_resize_begin_generic+0xc0>
33 addi r7,r9,560 30 addi r9,r9,560
34 li r8,1 31 li r10,1
35 ldarx r10,0,r7 32 ldarx r3,0,r9,1
36 or r6,r8,r10 33 or r8,r3,r10
37 stdcx. r6,0,r7 34 stdcx. r8,0,r9
38 bne- <ext4_resize_begin_generic+0x84> 35 bne- <ext4_resize_begin_ppc+0x78>
39 isync 36 isync
37 clrldi r3,r3,63
40 andi. r9,r10,1 38 addi r3,r3,-1
41 li r3,0 39 rlwinm r3,r3,0,27,27
42 bne <ext4_resize_begin_generic+0xc0> 40 addi r3,r3,-16
43 addi r1,r1,112 41 addi r1,r1,112
44 ld r0,16(r1) 42 ld r0,16(r1)
45 ld r30,-16(r1) 43 ld r30,-16(r1)
46 ld r31,-8(r1) 44 ld r31,-8(r1)
47 mtlr r0 45 mtlr r0
48 blr 46 blr
49 nop 47 nop
50 addi r1,r1,112 48 nop
51 li r3,-16
52 ld r0,16(r1)
53 ld r30,-16(r1)
54 ld r31,-8(r1)
55 mtlr r0
56 blr
57 nop 49 nop
58 li r3,-1 50 li r3,-1
59 b <ext4_resize_begin_generic+0xa4> 51 b <ext4_resize_begin_ppc+0x9c>
60 nop 52 nop
61 nop 53 nop
62 addis r6,r2,-118 54 addis r6,r2,-118
63 addis r4,r2,-140 55 addis r4,r2,-140
64 mr r3,r31 56 mr r3,r31
65 li r5,97 57 li r5,46
66 addi r6,r6,30288 58 addi r6,r6,30288
67 addi r4,r4,3064 59 addi r4,r4,3040
68 bl <__ext4_warning+0x8> 60 bl <__ext4_warning+0x8>
69 nop 61 nop
70 li r3,-1 62 li r3,-1
71 b <ext4_resize_begin_generic+0xa4> 63 b <ext4_resize_begin_ppc+0x9c>
72 ld r9,96(r9) 64 ld r9,96(r9)
73 addis r6,r2,-118 65 addis r6,r2,-118
74 addis r4,r2,-140 66 addis r4,r2,-140
75 mr r3,r31 67 mr r3,r31
76 li r5,87 68 li r5,36
77 addi r6,r6,30240 69 addi r6,r6,30240
78 addi r4,r4,3064 70 addi r4,r4,3040
79 ld r7,24(r9) 71 ld r7,24(r9)
80 bl <__ext4_warning+0x8> 72 bl <__ext4_warning+0x8>
81 nop 73 nop
82 li r3,-1 74 li r3,-1
83 b <ext4_resize_begin_generic+0xa4> 75 b <ext4_resize_begin_ppc+0x9c>


So READ_ONCE() + STACKPROTECTOR_STRONG is problematic. The root cause is
presumably that READ_ONCE() does an access to an on-stack variable,
which triggers the heuristics in the compiler that the stack needs
protecting.

It seems like a compiler "mis-feature" that a constant-sized access to the stack
triggers the stack protector logic, especially when the access is eventually
optimised away. But I guess that's probably what we get for doing tricks like
READ_ONCE() in the first place :/

I tried going back to the version of READ_ONCE() that doesn't use a
union, ie. effectively reverting dd36929720f4 ("kernel: make READ_ONCE()
valid on const arguments") to get:

#define READ_ONCE(x) \
({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; })

But it makes no difference, the stack protector stuff still triggers. So
I guess it's simply taking the address of a stack variable that triggers
it.

There seems to be a function attribute to enable stack protector for a
function, but not one to disable it:
https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/Common-Function-Attributes.html#index-stack_005fprotect-function-attribute

That may not be a good solution even if it did exist, because it would
potentially disable stack protector in places where we do want it
enabled.

cheers

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