Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()
From: Chen Zhongjin
Date: Thu Jun 30 2022 - 22:14:02 EST
Hi everyone,
Hope I'm not too late for this discussion.
I'm not familiar with ppc so it spent me some time to reproduce this.
But at last I didn't make it.
What I did:
1 checkout to tip/objtool/core
2 apply this patch
3 recover the unreachable() after WARN_ENTRY(..
4 compile on defconfig/allyesconfig
If anyone can point out which file will generate this problem it will be
perfect.
On 2022/6/30 16:05, Naveen N. Rao wrote:
Christophe Leroy wrote:
Hi Sathvika,
Adding ARM people as they seem to face the same kind of problem (see
https://patchwork.kernel.org/project/linux-kbuild/patch/20220623014917.199563-33-chenzhongjin@xxxxxxxxxx/)
For my situation, the problem is, if there is an unreachable() used in
the switch default case with nothing else, compiler will generate a NOP
and is still a jump to this NOP branch while it is marked in
.discard.unreachable.
When objtool deal with .discard.unreachable, it will *do nothing* to
this NOP itself, but mark the previous instruction as "dead_end" (see
check.c:ignore_unreachable_insn()). And checking will stop when reach
this dead_end insn.
0x4: jne 0x14 <= jump for switch case
..
0x10: ret <= dead_end
0x14: nop <= real unreachable instructiond
However, actually we have a jump to NOP, which makes this reachable to
this branch, and when this NOP is at end of function, it get a "fall
through" warning.
I changed the unreachable to -EINVAL but it was criticized by the
committer because he thought it is objtool's job to deal with these
special cases.
Le 27/06/2022 à 17:35, Sathvika Vasireddy a écrit :
On 25/06/22 12:16, Christophe Leroy wrote:
Le 24/06/2022 à 20:32, Sathvika Vasireddy a écrit :
objtool is throwing *unannotated intra-function call*
warnings with a few instructions that are marked
unreachable. Remove unreachable() from WARN_ON()
to fix these warnings, as the codegen remains same
with and without unreachable() in WARN_ON().
Did you try the two exemples described in commit 1e688dd2a3d6
("powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS()
with
asm goto") ?
Without your patch:
00000640 <test>:
640: 81 23 00 84 lwz r9,132(r3)
644: 71 29 40 00 andi. r9,r9,16384
648: 40 82 00 0c bne 654 <test+0x14>
64c: 80 63 00 0c lwz r3,12(r3)
650: 4e 80 00 20 blr
654: 0f e0 00 00 twui r0,0
00000658 <test9w>:
658: 2c 04 00 00 cmpwi r4,0
65c: 41 82 00 0c beq 668 <test9w+0x10>
660: 7c 63 23 96 divwu r3,r3,r4
664: 4e 80 00 20 blr
668: 0f e0 00 00 twui r0,0
66c: 38 60 00 00 li r3,0
670: 4e 80 00 20 blr
With your patch:
00000640 <test>:
640: 81 23 00 84 lwz r9,132(r3)
644: 71 29 40 00 andi. r9,r9,16384
648: 40 82 00 0c bne 654 <test+0x14>
64c: 80 63 00 0c lwz r3,12(r3)
650: 4e 80 00 20 blr
654: 0f e0 00 00 twui r0,0
658: 4b ff ff f4 b 64c <test+0xc> <==
0000065c <test9w>:
65c: 2c 04 00 00 cmpwi r4,0
660: 41 82 00 0c beq 66c <test9w+0x10>
664: 7c 63 23 96 divwu r3,r3,r4
668: 4e 80 00 20 blr
66c: 0f e0 00 00 twui r0,0
670: 38 60 00 00 li r3,0 <==
674: 4e 80 00 20 blr <==
678: 38 60 00 00 li r3,0
67c: 4e 80 00 20 blr
The builtin variant of unreachable (__builtin_unreachable()) works.
How about using that instead of unreachable() ?
In fact the problem comes from the macro annotate_unreachable() which
is called by unreachable() before calling __build_unreachable().
Seems like this macro adds (after the unconditional trap twui) a call
to an empty function whose address is listed in section
.discard.unreachable
1c78: 00 00 e0 0f twui r0,0
1c7c: 55 e7 ff 4b bl 3d0
<qdisc_root_sleeping_lock.part.0>
RELOCATION RECORDS FOR [.discard.unreachable]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_REL32 .text+0x00000000000003d0
The problem is that that function has size 0:
00000000000003d0 l F .text 0000000000000000
qdisc_root_sleeping_lock.part.0
And objtool is not prepared for a function with size 0.
annotate_unreachable() seems to have been introduced in commit
649ea4d5a624f0 ("objtool: Assume unannotated UD2 instructions are dead
ends").
Objtool considers 'ud2' instruction to be fatal, so BUG() has
__builtin_unreachable(), rather than unreachable(). See commit
bfb1a7c91fb775 ("x86/bug: Merge annotate_reachable() into _BUG_FLAGS()
asm"). For the same reason, __WARN_FLAGS() is annotated with
_ASM_REACHABLE so that objtool can differentiate warnings from a BUG().
On powerpc, we use trap variants for both and don't have a special
instruction for a BUG(). As such, for _WARN_FLAGS(), using
__builtin_unreachable() suffices to achieve optimal code generation
from the compiler. Objtool would consider subsequent instructions to
be reachable. For BUG(), we can continue to use unreachable() so that
objtool can differentiate these from traps used in warnings.
It is right and on arm64 only BUG() has unreachable and there is no
annotation for __WARN_FLAGS(). Objtool works correctly on this. For that
I support that unreachable() annotation shouldn't be with __WARN_FLAGS()
because there should be an accessible branch after WARN() micro. However
in the previous case, it's wired that compiler generates a bl to
unreachable symbol, IIUC it is not a illegal call? (if it is allowed on
ppc then objtool should be tell to recognize this)
It seems that your decoding only care about INSN_CALL for mcount, so
maybe temporarily these control flow checking makes non-sense for you so
the solution could actually be looser.
Anyway, maybe I can help more if I can reproduce that on my own machine.
Best,
Chen
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