gh-144759: Fix undefined behavior from NULL pointer arithmetic in lexer#144788
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pablogsal merged 3 commits intopython:mainfrom Feb 15, 2026
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gh-144759: Fix undefined behavior from NULL pointer arithmetic in lexer#144788pablogsal merged 3 commits intopython:mainfrom
pablogsal merged 3 commits intopython:mainfrom
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…in lexer Guard against NULL pointer arithmetic in `_PyLexer_remember_fstring_buffers` and `_PyLexer_restore_fstring_buffers`. When `start` or `multi_line_start` are NULL (uninitialized in tok_mode_stack[0]), performing `NULL - tok->buf` is undefined behavior. Add explicit NULL checks to store -1 as sentinel and restore NULL accordingly.
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…tions Replace :c:func: references with double-backtick markup since these are internal functions without documentation entries.
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@raminfp Could you add a regression test? I suspect And please avoid force pushes to the PR so we preserve history. |
…exer Add test_lexer_buffer_realloc_with_null_start to test_repl.py that exercises the code path where the lexer buffer is reallocated while tok_mode_stack[0] has NULL start/multi_line_start pointers. This triggers _PyLexer_remember_fstring_buffers and verifies the NULL checks prevent undefined behavior.
pablogsal
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Great catch! |
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needs backport to 3.13
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Thanks @raminfp for the PR, and @pablogsal for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14. |
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…in lexer (pythonGH-144788) Guard against NULL pointer arithmetic in `_PyLexer_remember_fstring_buffers` and `_PyLexer_restore_fstring_buffers`. When `start` or `multi_line_start` are NULL (uninitialized in tok_mode_stack[0]), performing `NULL - tok->buf` is undefined behavior. Add explicit NULL checks to store -1 as sentinel and restore NULL accordingly. Add test_lexer_buffer_realloc_with_null_start to test_repl.py that exercises the code path where the lexer buffer is reallocated while tok_mode_stack[0] has NULL start/multi_line_start pointers. This triggers _PyLexer_remember_fstring_buffers and verifies the NULL checks prevent undefined behavior. (cherry picked from commit e6110ef) Co-authored-by: Ramin Farajpour Cami <ramin.blackhat@gmail.com>
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Sorry, @raminfp and @pablogsal, I could not cleanly backport this to |
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GH-144834 is a backport of this pull request to the 3.14 branch. |
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@raminfp can you follow these instructions to create the 3.13 back port #144788 (comment) thanks! |
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… in lexer (GH-144788) (#144834) gh-144759: Fix undefined behavior from NULL pointer arithmetic in lexer (GH-144788) Guard against NULL pointer arithmetic in `_PyLexer_remember_fstring_buffers` and `_PyLexer_restore_fstring_buffers`. When `start` or `multi_line_start` are NULL (uninitialized in tok_mode_stack[0]), performing `NULL - tok->buf` is undefined behavior. Add explicit NULL checks to store -1 as sentinel and restore NULL accordingly. Add test_lexer_buffer_realloc_with_null_start to test_repl.py that exercises the code path where the lexer buffer is reallocated while tok_mode_stack[0] has NULL start/multi_line_start pointers. This triggers _PyLexer_remember_fstring_buffers and verifies the NULL checks prevent undefined behavior. (cherry picked from commit e6110ef) Co-authored-by: Ramin Farajpour Cami <ramin.blackhat@gmail.com>
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Fix undefined behavior in
_PyLexer_remember_fstring_buffersand_PyLexer_restore_fstring_bufferscaused by performing pointer arithmetic onNULLpointers (NULL - tok->buf).When
tok_mode_stack[0]is initialized, thestartandmulti_line_startfields are not explicitly set and remainNULL(fromPyMem_Calloc). Later, when the lexer buffer is reallocated, the remember/restore functions performNULL - valid_pointerandvalid_pointer + negative_offset, both of which are undefined behavior in C.The fix adds explicit
NULLchecks: store-1as a sentinel offset when the pointer isNULL, and restoreNULLwhen the offset is negative.Detected with
--with-undefined-behavior-sanitizer:Fixes #144759