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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Memory sanitizer generates false positives on data returned by os.getrandom()","articleBody":"# Bug report\n\n### Bug description:\n\nWhen python is built with memory sanitizer enabled, reading any data returned by `os.getrandom()` will trigger a memory sanitizer false positive, because the buffer is filled using a syscall, and the memory isn't marked as having been written to.\n\nCurrently, \n```\n$ cat test.py \nimport os\ndata = os.getrandom(32)\nh = data.hex()\nprint(f'h={h}')\n\n$ ./python ./test.py \nUninitialized bytes in memchr at offset 18 inside [0x707000006458, 66)\n==242471==WARNING: MemorySanitizer: use-of-uninitialized-value\n...\nSUMMARY: MemorySanitizer: use-of-uninitialized-value /home/peter/dev/pvanvugt-cpython/Objects/stringlib/fastsearch.h:58:13 in ucs1lib_find_char\nExiting\n```\n\nI have verified that applying `__masan_unpoison()` to the data fixes this.\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-148851\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/petervanvugt","@type":"Person","name":"petervanvugt"},"datePublished":"2026-04-21T19:01:53.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/148850/cpython/issues/148850"}