linux-kernel - Re: [PATCH v3 2/3] mmap: Fix error paths with dup_anon

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Date:   Mon, 2 Oct 2023 09:26:03 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Lorenzo Stoakes <lstoakes@...il.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH v3 2/3] mmap: Fix error paths with dup_anon_vma()

On 9/30/23 00:28, Lorenzo Stoakes wrote:
> On Fri, Sep 29, 2023 at 02:30:40PM -0400, Liam R. Howlett wrote:
>> When the calling function fails after the dup_anon_vma(), the
>> duplication of the anon_vma is not being undone.  Add the necessary
>> unlink_anon_vma() call to the error paths that are missing them.
>>
>> This issue showed up during inspection of the error path in vma_merge()
>> for an unrelated vma iterator issue.
>>
>> Users may experience increased memory usage, which may be problematic as
>> the failure would likely be caused by a low memory situation.
>>
>> Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
>> Cc: stable@...r.kernel.org
>> Cc: Jann Horn <jannh@...gle.com>
>> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
>> ---
>>  mm/mmap.c | 30 ++++++++++++++++++++++--------
>>  1 file changed, 22 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index acb7dea49e23..f9f0a5fe4db4 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -583,11 +583,12 @@ static inline void vma_complete(struct vma_prepare *vp,
>>   * dup_anon_vma() - Helper function to duplicate anon_vma
>>   * @dst: The destination VMA
>>   * @src: The source VMA
>> + * @dup: Pointer to the destination VMA when successful.
>>   *
>>   * Returns: 0 on success.
> 
> Being a bit nitpicky/refactory here, but anon_vma_clone() appears to have
> two possible return values - 0 for success, and -ENOMEM.
> 
> As a result, it's not really gaining us much passing through this value.
> 
> It'd be nice if dup_anon_vma() and anon_vma_clone() were therefore updated
> to instead return NULL on ENOMEM and the dst otherwise.

But we also need to represent that dup_anon_vma() had nothing to do, because
"(src->anon_vma && !dst->anon_vma)" was false, and in that case we should
not be returning dst from there?

So maybe we could return NULL for that case and ERR_PTR(ret) for the -ENOMEM
from anon_vma_clone() ?

> Then we could de-clunk this whole code path, and the quite natural fact of
> 'thing didn't return a pointer therefore had no memory to allocate it' fals
> out.
> 
> But this isn't exactly an earth-shattering concern :)
>