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List-Unsubscribe: On Tue, Jun 17, 2025 at 05:21:03PM +0800, Lance Yang wrote: > > > On 2025/6/17 16:50, Lorenzo Stoakes wrote: > > Lace - To simplify and not get bogged down in sub-threads am replying at the top > > level. Firstly let me profusely apologise for calling you 'Lace' LOL! ;) Busy morning and typos are happening... > > > > TL;DR this fix is incorrect, but the issue is correct :) > > > > So the patch at [0] introduced by Barry changed things in a way that _appears_ > > broken but in fact aren't, however we should do something about this, obviously. > > > > That patch added: > > > > if (madv_behavior && madv_behavior->lock_mode == MADVISE_VMA_READ_LOCK) { > > vma = try_vma_read_lock(mm, madv_behavior, start, end); > > if (vma) { > > error = visit(vma, &prev, start, end, arg); > > vma_end_read(vma); > > return error; > > } > > } > > > > And the problem is, in this case, we don't initialise prev. > > > > In all other cases, we do (under mmap lock): > > > > vma = find_vma_prev(mm, start, &prev); > > if (vma && start > vma->vm_start) > > prev = vma; > > > > The reason this isn't a problem is that the only madvise operation that > > currently supports this, madvise_dontneed_free() will initialise *prev = vma. > > > > BUT we really shouldn't be relying on this, so I attach a fixpatch. > > > > Given Barry's patch isn't mainline yet, I think this should just be squashed > > into that as a fix? > > > > It kind of sucks to do this, but it resolves any potential bug. > > > > I think a follow up is needed, as there's an implicit assumption it seems that > > prev is updated immediately for most callers, but of course anon_vma_name is a > > special snowflake. > > > > todos++; > > Ah, please keep me in the loop ;) Will do! Thanks for finding this issue, is appreciated! > > > > > Lance - I suggest you reply to Barry's series with the below fix, or I can if > > you prefer? > > Sure, go ahead! Thanks will chase up! Cheers, Lorenzo > > Thanks, > Lance > > > > > [0]: https://lore.kernel.org/linux-mm/20250607220150.2980-1-21cnbao@gmail.com/ > > > > Thanks! > > > > On Tue, Jun 17, 2025 at 10:05:43AM +0800, Lance Yang wrote: > > > From: Lance Yang > > > > > > The prev pointer was uninitialized, which could lead to undefined behavior > > > where its address is taken and passed to the visit() callback without being > > > assigned a value. > > > > > > Initializing it to NULL makes the code safer and prevents potential bugs > > > if a future callback function attempts to read from it. > > > > > > Signed-off-by: Lance Yang > > > --- > > > mm/madvise.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > > index 267d8e4adf31..c87325000303 100644 > > > --- a/mm/madvise.c > > > +++ b/mm/madvise.c > > > @@ -1536,10 +1536,10 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start, > > > struct vm_area_struct **prev, unsigned long start, > > > unsigned long end, void *arg)) > > > { > > > + struct vm_area_struct *prev = NULL; > > > struct vm_area_struct *vma; > > > - struct vm_area_struct *prev; > > > - unsigned long tmp; > > > int unmapped_error = 0; > > > + unsigned long tmp; > > > int error; > > > > > > /* > > > -- > > > 2.49.0 > > > > > > > ----8<---- > > From c8dc9f5b2929e389cac44b79201fff43e0ab8195 Mon Sep 17 00:00:00 2001 > > From: Lorenzo Stoakes > > Date: Tue, 17 Jun 2025 09:46:27 +0100 > > Subject: [PATCH] fix > > > > Signed-off-by: Lorenzo Stoakes > > --- > > mm/madvise.c | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > index 267d8e4adf31..45ea4588e34e 100644 > > --- a/mm/madvise.c > > +++ b/mm/madvise.c > > @@ -1549,6 +1549,7 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start, > > if (madv_behavior && madv_behavior->lock_mode == MADVISE_VMA_READ_LOCK) { > > vma = try_vma_read_lock(mm, madv_behavior, start, end); > > if (vma) { > > + *prev = vma; > > error = visit(vma, &prev, start, end, arg); > > vma_end_read(vma); > > return error; > > -- > > 2.49.0 >