jasonmolenda added a comment.
Ah, I can't share the binary/corefile that led me to this issue but I can show
what it looks like. I have a function with a noreturn call, it's a crashing
scenario where (as always) this is the most important bit to get right, to show
the crash,
0xfffffff02058d2c4 <+3404>: mov x1, x19
0xfffffff02058d2c8 <+3408>: bl -0xfdf3b49dc ;
panic_with_thread_kernel_state
-> 0xfffffff02058d2cc <+3412>: mov x0, x19
0xfffffff02058d2d0 <+3416>: mov x1, x24
and the unwind plan rules for these instructions look like
row[115]: 3396: CFA=fp+16 => x5= <same> x8= <same> x19=[CFA-24] x20=[CFA-32]
x21=[CFA-40] x22=[CFA-48] x23=[CFA-56] x24=[CFA-64] x25=[CFA-72] x26=[CFA-80]
x27=[CFA-88] x28=[CFA-96] fp=[CFA-16] lr=[CFA-8]
row[116]: 3412: CFA=fp+16 => x19= <same> x20= <same> x21= <same> x22= <same>
x23= <same> x24= <same> x25= <same> x26= <same> x27= <same> x28= <same> fp=
<same> lr= <same>
row[117]: 3428: CFA=fp+16 => x8=[CFA-128] x19=[CFA-24] x20=[CFA-32]
x21=[CFA-40] x22=[CFA-48] x23=[CFA-56] x24=[CFA-64] x25=[CFA-72] x26=[CFA-80]
x27=[CFA-88] x28=[CFA-96] fp=[CFA-16] lr=[CFA-8]
We're under the BL call here, and if we use
row[115]: 3396: CFA=fp+16 => x5= <same> x8= <same> x19=[CFA-24] x20=[CFA-32]
x21=[CFA-40] x22=[CFA-48] x23=[CFA-56] x24=[CFA-64] x25=[CFA-72] x26=[CFA-80]
x27=[CFA-88] x28=[CFA-96] fp=[CFA-16] lr=[CFA-8]
we'll walk the stack correctly. But instead we're using the return address
which has an unwind rule
row[116]: 3412: CFA=fp+16 => x19= <same> x20= <same> x21= <same> x22= <same>
x23= <same> x24= <same> x25= <same> x26= <same> x27= <same> x28= <same> fp=
<same> lr= <same>
(which is honestly real suspect in its own right and I'll be looking at what
codepath lldb was following when it came up with that, it's a complex function
and I don't have an opinion at first blush)
But the problem is obvious when you see this - the unwind state after a
noreturn could be any old thing, and we can't use it reliably.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D124957/new/
https://reviews.llvm.org/D124957
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