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 LgWgLukq 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Howlett wrote: > > * Vlastimil Babka [241031 13:07]: > > > On 10/31/24 18:01, Liam R. Howlett wrote: > > > > From: "Liam R. Howlett" > > > > > > > > There have been no reported infinite loops in the tree, but checking the > > > > detection of an infinite loop during validation is simple enough. Add > > > > the detection to the validate_mm() function so that error reports are > > > > clear and don't just report stalls. > > > > > > > > This does not protect against internal maple tree issues, but it does > > > > detect too many vmas being returned from the tree. > > > > > > > > Signed-off-by: Liam R. Howlett > > > > Cc: Lorenzo Stoakes > > > > Cc: Vlastimil Babka > > > > Cc: Jann Horn > > > > --- > > > > mm/vma.c | 3 ++- > > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/mm/vma.c b/mm/vma.c > > > > index 68138e8c153e..60ed8cc187ad 100644 > > > > --- a/mm/vma.c > > > > +++ b/mm/vma.c > > > > @@ -615,7 +615,8 @@ void validate_mm(struct mm_struct *mm) > > > > anon_vma_unlock_read(anon_vma); > > > > } > > > > #endif > > > > - i++; > > > > + if (++i > mm->map_count) > > > > + break; > > > > > > Would it make sense to allow some slack so that the error below can > > > distinguish better between off-by-one/few error from a complete corruption? > > > > > > And in that case assign some special value to "i" (-1?) to make it clear > > > this was triggered? > > > > Yes, probably. 10 would be plenty. In recent memory I cannot think of > > an example that we exceeded 7 munmap()'s in a single operation - > > although it is easily possible to do. > > > > I like the idea of -1 too, at least someone would come to inspect where > > it came from at that point. > > Hm this feels a little arbitrary though... I mean can we race with > map_count at this point or is everything locked down such that no munmaps() > can happen? > > Otherwise it feels a bit whack-a-mole. > > I agree with Vlastimil though it'd be nice to sort of differentiate, but if > we _absolutely can only iterate mm->map_count times_ here, it might be > worth letting a few more go, then in the next bit of code... this is done under the write lock, otherwise it would vary already. > > > > > > > > > > } > > > > if (i != mm->map_count) { > > > > pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i); > > > > > ...here which does indeed imply that i literally cannot be anything but > mm->map_count, I mean I guess we already get to see here how far off we > are. > > So yeah something like letting it go 10 more times (maybe like #define > UNUSUALLY_BAD_CORRUPTION_COUNT 10 or something I don't know naming is hard) > just so we can pick that out would be nice. > > But I do like the general idea here though! This may not ever produce anything - if we are in the maple tree code and never reaching a leaf to return anything then we will still loop forever. But it is something that is easy enough to detect and stop - which may make a syzbot report a bit easier to swallow.