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Date: Sun, 30 Aug 2020 17:34:09 -0700
From: Theodore Dubois <tblodt@...oud.com>
To: musl@...ts.openwall.com
Subject: i386 __set_thread_area will crash if the syscall fails
Found a (small) bug in this file: https://git.musl-libc.org/cgit/musl/tree/src/thread/i386/__set_thread_area.s
If the syscall fails, the branch on line 20 is taken and %eax will be a small negative number. Then "mov $123,%al" will make syscall 0xffffff7b instead of 0x7b, since overwriting %al only overwrites the low byte of %eax. So the modify_ldt fallback has apparently never worked.
Tangentially, I'm not sure why this file has so many hardcoded magic numbers and no comments to explain what they are.
~Theodore
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