Stream: git-wasmtime

Topic: wasmtime / PR #7590 Winch: cleanup stack in br_if in non-...

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:07):

jeffcharles requested elliottt for a review on PR #7590.

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:07):

jeffcharles requested wasmtime-compiler-reviewers for a review on PR #7590.

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:07):

jeffcharles opened PR #7590 from jeffcharles:winch-br-if-stack-cleanup to bytecodealliance:main:

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I discovered that executing the following module compiled with Winch causes a segmentation fault:

(module
  (func (export "")
    call 1
    call 1
    br_if 0
    drop
  )
  (func (;1;) (result i32)
    i32.const 1
  )
)

The cause appears to be that the code that would increment rsp to balance the machine stack is jumped over when the br_if path is taken.

What appears to work is, if the frame's sp_offset is different than the current sp_offset, we invert the existing jump and execute instructions to balance the stack before jumping to the frame's label, and reset the sp_offset before continuing with the fallthrough code (falling through in terms of the br_if, not the machine code jump). In the case where there is no difference between sp_offsets, we can use the existing jne machine code emission since we don't need to run any stack balancing instructions.

In terms of machine instructions, we'd be changing

sub rsp, 4
test <reg> <reg>
jne end
add rsp, 4
end:
add rsp, 8
pop rbp
ret

to

sub rsp, 4
test <reg> <reg>
jeq fallthrough
add rsp, 4
jmp end
fallthrough:
add rsp, 4
end:
add rsp, 8
pop rbp
ret

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:11):

jeffcharles submitted PR review.

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:11):

jeffcharles created PR review comment:

The old file test instructions resulted in an imbalanced stack, these new instructions are balanced. I suspect visiting the unreachable instruction may have put Winch into a state where it did not validate the sp_offset at the end was equal to the locals size.

Old:

;;    0:     55                     push    rbp
;;    1:     4889e5                 mov rbp, rsp
;;    4:     4883ec10               sub rsp, 0x10                    (offset += 16, 16)
;;    8:     897c240c               mov dword ptr [rsp + 0xc], edi
;;    c:     4c893424               mov qword ptr [rsp], r14
;;   10:     448b5c240c             mov r11d, dword ptr [rsp + 0xc]
;;   15:     4883ec04               sub rsp, 4                       (offset += 4, 20)
;;   19:     44891c24               mov dword ptr [rsp], r11d
;;   1d:     4883ec0c               sub rsp, 0xc                     (offset += 12, 32)
;;   21:     bf01000000             mov edi, 1
;;   26:     e800000000             call    0x2b
;;   2b:     4883c40c               add rsp, 0xc                     (offset -= 12, 20)
;;   2f:     4883ec04               sub rsp, 4                       (offset += 4, 24)
;;   33:     890424                 mov dword ptr [rsp], eax
;;   36:     4883ec08               sub rsp, 8                       (offset += 8, 32)
;;   3a:     bf01000000             mov edi, 1
;;   3f:     e800000000             call    0x44
;;   44:     4883c408               add rsp, 8                       (offset -= 8, 24)
;;   48:     4883ec04               sub rsp, 4                       (offset += 4, 28)
;;   4c:     890424                 mov dword ptr [rsp], eax
;;   4f:     8b0c24                 mov ecx, dword ptr [rsp]
;;   52:     4883c404               add rsp, 4                       (offset -= 4, 24)
;;   56:     8b0424                 mov eax, dword ptr [rsp]
;;   59:     4883c404               add rsp, 4                       (offset -= 4, 20)
;;   5d:     85c9                   test    ecx, ecx
;;   5f:     0f8502000000           jne 0x67
;;   65:     0f0b                   ud2
;;   67:     4883c410               add rsp, 0x10                    (offset -= 16, 4)
;;   6b:     5d                     pop rbp
;;   6c:     c3                     ret

New:

;;    0:     55                     push    rbp
;;    1:     4889e5                 mov rbp, rsp
;;    4:     4883ec10               sub rsp, 0x10                    (offset += 16, 16)
;;    8:     897c240c               mov dword ptr [rsp + 0xc], edi
;;    c:     4c893424               mov qword ptr [rsp], r14
;;   10:     448b5c240c             mov r11d, dword ptr [rsp + 0xc]
;;   15:     4883ec04               sub rsp, 4                       (offset += 4, 20)
;;   19:     44891c24               mov dword ptr [rsp], r11d
;;   1d:     4883ec0c               sub rsp, 0xc                     (offset += 12, 32)
;;   21:     bf01000000             mov edi, 1
;;   26:     e800000000             call    0x2b
;;   2b:     4883c40c               add rsp, 0xc                     (offset -= 12, 20)
;;   2f:     4883ec04               sub rsp, 4                       (offset += 4, 24)
;;   33:     890424                 mov dword ptr [rsp], eax
;;   36:     4883ec08               sub rsp, 8                       (offset += 8, 32)
;;   3a:     bf01000000             mov edi, 1
;;   3f:     e800000000             call    0x44
;;   44:     4883c408               add rsp, 8                       (offset -= 8, 24)
;;   48:     4883ec04               sub rsp, 4                       (offset += 4, 28)
;;   4c:     890424                 mov dword ptr [rsp], eax
;;   4f:     8b0c24                 mov ecx, dword ptr [rsp]
;;   52:     4883c404               add rsp, 4                       (offset -= 4, 24)
;;   56:     8b0424                 mov eax, dword ptr [rsp]
;;   59:     4883c404               add rsp, 4                       (offset -= 4, 20)
;;   5d:     85c9                   test    ecx, ecx
;;   5f:     0f8409000000           je  0x6e
;;   65:     4883c404               add rsp, 4                       (offset -= 4, 16)
;;   69:     e902000000             jmp 0x70
;;   6e:     0f0b                   ud2
;;   70:     4883c410               add rsp, 0x10                    (offset -= 16, 0)
;;   74:     5d                     pop rbp
;;   75:     c3                     ret

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:22):

jeffcharles updated PR #7590.

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:35):

saulecabrera submitted PR review.

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:35):

saulecabrera submitted PR review.

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:35):

saulecabrera created PR review comment:

Minor note here: instead of checking for equality, can we check for current_sp_offset > frame_sp_offset? We have checks everywhere else that guarantee that the current sp should never be less than the target SP; doing so will also align with how ensure_sp_for_jump is built and it also conveys a clearer message that the stack will be manipulated.

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:35):

saulecabrera created PR review comment:

It seems that we can avoid some repetition here, as only the label and the CmpKind are different for the branch instruction:

let (label, cmp, needs_cleanup) = if current_sp_offset > frame_sp_offset {
    (masm.get_label(), IntCmpKind::Eq, true)
} else {
    (•frame.label(), IntCmpKind::Ne, false)
};

self.masm.branch(...);

if needs_cleanup { .. }

Wasmtime GitHub notifications bot (Nov 27 2023 at 23:35):

saulecabrera edited PR review comment.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:43):

jeffcharles submitted PR review.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:43):

jeffcharles created PR review comment:

Should SPOffset implement Ord? I can call .as_u32() on both sides, but I _think_ it would be reasonable to be able to compare them directly.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:53):

jeffcharles updated PR #7590.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:54):

jeffcharles requested saulecabrera for a review on PR #7590.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:57):

saulecabrera submitted PR review.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:57):

saulecabrera created PR review comment:

Yeah, I think it's fine for it to implement Ord.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:57):

saulecabrera submitted PR review.

Wasmtime GitHub notifications bot (Nov 28 2023 at 16:57):

saulecabrera created PR review comment:

We can do it in a follow-up PR too.

Wasmtime GitHub notifications bot (Nov 28 2023 at 18:33):

jeffcharles updated PR #7590.

Wasmtime GitHub notifications bot (Nov 28 2023 at 20:05):

saulecabrera submitted PR review.

Wasmtime GitHub notifications bot (Nov 28 2023 at 20:53):

saulecabrera merged PR #7590.

Last updated: Oct 23 2024 at 20:03 UTC