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UB: (int) cast of a user double outside int range in dispatch / vm_index_is_int / flat_index_at

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c
Domain
compilers

Research direction

Start with the four named sites: builtin_dispatch in src/builtins.c around line 5946, vm_index_is_int in src/vm.c, the list arm of flat_index_at in src/builtins_tensor.c, and finite_num_to_byte. Reproduce with the dispatch snippet from the issue under make asan to see the UBSan float-cast-overflow report. Done when each site range-checks the double before casting (NaN-false), the grep -n '(int)' src/*.c sweep is classified, and the repro prints the value error with no runtime error: line.

Written by the indexing model from the issue text.

Description

area:runtime-vm

Found while building #1746 Phase 1. Several sites convert a user double to int with a bare (int) cast before any range check. For |x| >= 2^31 (today: up to ±1e308; under an IEEE build also inf/NaN) that is undefined behaviour, and UBSan's float-cast-overflow (on in every ASan variant) reports it.

Repro (any asan build):

define g(x) as:
    return x
d is dispatch
print of (d of [[g], 1e308, 5])

-> src/builtins.c:5946:20: runtime error: 1e+308 is outside the range of representable values of type 'int' (then the intended dispatch key must be an integer raise).

Sites: builtin_dispatch (builtins.c, (int)eigs_num_arg(key_v, ...)), vm_index_is_int (vm.c), flat_index_at list arm (builtins_tensor.c), finite_num_to_byte ((int)fmod(x,256) with a NaN). The #1746 experiment guards only vm_index_is_int, only under -DEIGS_IEEE.

Done when

  • Each listed site range-checks (!(d >= INT_MIN && d <= INT_MAX) style, NaN-false) before the cast, and the sweep grep -n '(int)' src/*.c over user-double conversions is classified.
  • The repro above is clean under make asan (no runtime error:), and still raises value.
Dominant language
C
Stars
3
Forks
7
Avg merge
8h 43m
Merged PRs (30d)
253

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