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frame.setlineno has serious flaws. #94438

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markshannon opened this issue Jun 30, 2022 · 0 comments
Open
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frame.setlineno has serious flaws. #94438

markshannon opened this issue Jun 30, 2022 · 0 comments
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@iritkatriel @markshannon
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3.11 3.12 deferred-blocker type-crash

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@markshannon markshannon commented Jun 30, 2022 •
edited by iritkatriel

The frame_setlineno function works in in stages:

  • Determine a set of possible bytecode offsets as targets from the line number.
  • Compute the stack state for these targets and the current position
  • Determine a best target. That is, the first one that has a compatible stack.
  • Pop values form the stack and jump.

The first steps is faulty (I think, I haven't demonstrated this) as it might be possible to jump to an instruction involved in frame creation. This should be easy to fix using the new _co_firsttraceable field.

The second step has (at least) three flaws:

  • It does not account for NULLs on the stack, making it possible to jump from a stack with NULLs to one that cannot handle NULLs.
  • It does not skip over caches, so could produce incorrect stacks by misinterpreting cache entries as normal instructions.
  • It is out of date. For example it thinks that PUSH_EXC_INFO pushes three values. It only pushes one.

Setting the line number of a frame is only possible in the debugger, so this isn't as terrible as might appear, but it definitely needs fixing.
@markshannon markshannon added type-crash deferred-blocker labels Jun 30, 2022
@bedevere-bot bedevere-bot mentioned this issue Jun 30, 2022
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@bedevere-bot bedevere-bot mentioned this issue Jul 1, 2022
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miss-islington pushed a commit to miss-islington/cpython that referenced this issue Jul 1, 2022
@markshannon @miss-islington
pythonGH-94438: Account for NULLs on evaluation stack when jumping li…
edfdd6e 
…nes. (pythonGH-94444)

(cherry picked from commit be80db14c432c621e44920f8fd95a3f3191aca9b)

Co-authored-by: Mark Shannon <mark@hotpy.org>
markshannon added a commit that referenced this issue Jul 1, 2022
@markshannon
GH-94438: Account for NULLs on evaluation stack when jumping lines. (G…
be80db1 
…H-94444)
@bedevere-bot bedevere-bot mentioned this issue Jul 1, 2022
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markshannon added a commit that referenced this issue Jul 1, 2022
@markshannon
[3.11] GH-94438: Backport GH-94444 (#94486)
02b30a8 
* Account for NULLs on evaluation stack when jumping lines.
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