Fix syz_open_dev virtual file names range #6068
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We noticed that syzkaller left some files with fairly unusual file names under /dev. Eg: ---------- 1 root root 0 May 30 14:42 vcs- ---------- 1 root root 0 May 30 14:48 vcs. ---------- 1 root root 136317631 May 30 14:42 vcs' ---------- 1 root root 0 May 30 14:48 vcs( ---------- 1 root root 0 May 30 14:43 vcs) ---------- 1 root root 0 May 30 14:43 vcs* ---------- 1 root root 136317633 May 30 14:46 vcs+ Funnily enough the characters after "vcs" are always within the '0'-10 to '0' ASCII range. We noticed that the syz_open_dev logic uses a modulo 10 on a signed number (the volatile long a1 argument) and in C the modulo of a negative number stays negative, so the result of this operation is in the '0'-10 to '0'+10 range. This is in turn casted to a char which is also signed and doesn't fix the glitch. By casting a1 to an unsigned long first, this keeps the result of the modulo operation signed and therefore the virtual file name suffix a number.
The logic in that branch of the code relies on replacing # characters with numbers. There's a comment in the code which shows a clarifying example but it misses the # which I found mildly confusing.
dvyukov
approved these changes
Jun 3, 2025
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