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authorPeter Zijlstra2020-04-28 19:37:01 +0200
committerPeter Zijlstra2020-04-30 20:14:31 +0200
commit7117f16bf460ef8cd132e6e80c989677397b4868 (patch)
tree0f7bf3d56c7af7cda6aa419109bb48deb1602d57 /tools/objtool/Documentation
parent13fab06d9a3ad3afdfd51c7f8f87f2ae28444648 (diff)
objtool: Fix ORC vs alternatives
Jann reported that (for instance) entry_64.o:general_protection has very odd ORC data: 0000000000000f40 <general_protection>: #######sp:sp+8 bp:(und) type:iret end:0 f40: 90 nop #######sp:(und) bp:(und) type:call end:0 f41: 90 nop f42: 90 nop #######sp:sp+8 bp:(und) type:iret end:0 f43: e8 a8 01 00 00 callq 10f0 <error_entry> #######sp:sp+0 bp:(und) type:regs end:0 f48: f6 84 24 88 00 00 00 testb $0x3,0x88(%rsp) f4f: 03 f50: 74 00 je f52 <general_protection+0x12> f52: 48 89 e7 mov %rsp,%rdi f55: 48 8b 74 24 78 mov 0x78(%rsp),%rsi f5a: 48 c7 44 24 78 ff ff movq $0xffffffffffffffff,0x78(%rsp) f61: ff ff f63: e8 00 00 00 00 callq f68 <general_protection+0x28> f68: e9 73 02 00 00 jmpq 11e0 <error_exit> #######sp:(und) bp:(und) type:call end:0 f6d: 0f 1f 00 nopl (%rax) Note the entry at 0xf41. Josh found this was the result of commit: 764eef4b109a ("objtool: Rewrite alt->skip_orig") Due to the early return in validate_branch() we no longer set insn->cfi of the original instruction stream (the NOPs at 0xf41 and 0xf42) and we'll end up with the above weirdness. In other discussions we realized alternatives should be ORC invariant; that is, due to there being only a single ORC table, it must be valid for all alternatives. The easiest way to ensure this is to not allow any stack modifications in alternatives. When we enforce this latter observation, we get the property that the whole alternative must have the same CFI, which we can employ to fix the former report. Fixes: 764eef4b109a ("objtool: Rewrite alt->skip_orig") Reported-by: Jann Horn <jannh@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Miroslav Benes <mbenes@suse.cz> Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> Link: https://lkml.kernel.org/r/20200428191659.499074346@infradead.org
Diffstat (limited to 'tools/objtool/Documentation')
-rw-r--r--tools/objtool/Documentation/stack-validation.txt7
1 files changed, 7 insertions, 0 deletions
diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
index faa47c3aafae..0189039489e9 100644
--- a/tools/objtool/Documentation/stack-validation.txt
+++ b/tools/objtool/Documentation/stack-validation.txt
@@ -315,6 +315,13 @@ they mean, and suggestions for how to fix them.
function tracing inserts additional calls, which is not obvious from the
sources).
+10. file.o: warning: func()+0x5c: alternative modifies stack
+
+ This means that an alternative includes instructions that modify the
+ stack. The problem is that there is only one ORC unwind table, this means
+ that the ORC unwind entries must be valid for each of the alternatives.
+ The easiest way to enforce this is to ensure alternatives do not contain
+ any ORC entries, which in turn implies the above constraint.
If the error doesn't seem to make sense, it could be a bug in objtool.
Feel free to ask the objtool maintainer for help.