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Check the index hasn't changed after writing the cmp entry.

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If an interrupt fires while writing the cmp entry we may have a partial
entry. Work around this by using atomic_cmpset to set the new index. If it
fails we need to set the previous index value and try again as the entry
may be in an inconsistent state.

This fixes messages similar to the following from syzkaller:
bad comp 224 type 2163727253

Reviewed by:	tuexen
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D19287
This commit is contained in:
Andrew Turner 2019-02-25 13:15:34 +00:00
parent 352a2062c9
commit feb2cc805f
1 changed files with 10 additions and 5 deletions
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15
sys/kern/kern_kcov.c
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@ -247,11 +247,16 @@ trace_cmp(uint64_t type, uint64_t arg1, uint64_t arg2, uint64_t ret)
if (index * 4 + 4 + 1 > info->entries)
return (false);
buf[index * 4 + 1] = type;
buf[index * 4 + 2] = arg1;
buf[index * 4 + 3] = arg2;
buf[index * 4 + 4] = ret;
buf[0] = index + 1;
while (1) {
buf[index * 4 + 1] = type;
buf[index * 4 + 2] = arg1;
buf[index * 4 + 3] = arg2;
buf[index * 4 + 4] = ret;
if (atomic_cmpset_64(&buf[0], index, index + 1))
break;
buf[0] = index;
}
return (true);
}