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(p200300d82f1ded001769dd7c7208eb33.dip0.t-ipconnect.de. [2003:d8:2f1d:ed00:1769:dd7c:7208:eb33]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4562e8860cdsm24478335e9.20.2025.07.16.08.18.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 16 Jul 2025 08:18:04 -0700 (PDT) Message-ID: <955c523e-9eea-48fb-a604-9ec445e563f4@redhat.com> Date: Wed, 16 Jul 2025 17:18:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] mm/huge_memory: prevent potential NULL pointer dereference To: Zi Yan , Antonio Quartulli Cc: linux-mm@kvack.org, Andrew Morton , Lorenzo Stoakes , Baolin Wang , "Liam R. 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: DD93980012 X-Stat-Signature: 8uox7b9cp97soagnca8jwxp1otap3jf9 X-Rspam-User: X-HE-Tag: 1752679088-702307 X-HE-Meta: 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 UgneWDgH 4s8EtPK/dCyEOwkvNQKOmJmXXFEHs8UvE+928y/xhjepRFVVUAzElYjX6Au8ypqRLdCWqORp5EkaJITAyEpuEQ6l56UMRejUYJlcUL5/15tnilBhL10BJb2scJowByGYcWetq9u6EF42qPDibwN12OgVMPZIUb85kZ+mgRaybV6AOEA7aby6I4BuUnY/4g+q1QEeME0ahOwmQH/g0DF4zjrcX+NLb7Ucy9fyYIBX4dnLEpmwrRVJD9tmQODch0BDn67jyWeQ5dNt7jn1iDcrF04e7rpizv+Smz0yLjG/qg3TkdqzzdCvOZEEHyJMnYvC5F6aS5Pds5R3gYOk6k2/afe5gmWsDDiOyrRrF5yhX0S/vOs2ssT8C8ukWpJWP6jhnv3/RjpdzXIzQzgcgfDCrnD8iG5gprhdWsnD6imXv0mmdx19F9WC3i3/sDpqkiYb5ps40OUnDlHbgxGLv3V2kvp6w9uOxvjiNXFOrK1R2Y/ewVoUJRGlDe8REhuaXtJRo7eBwSwNyy2R+ZfkbHRxfHel1n4k3QkBfJuLTISWrUuGtRclaexAlL+cp2w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 16.07.25 17:10, Zi Yan wrote: > On 16 Jul 2025, at 10:58, Antonio Quartulli wrote: > >> I just found this issue in the last linux-next Coverity report and it >> caught my attention. >> I am not familiar with this code, therefore I am sending this patch >> as RFC because I am not 100% sure whether this is a false positive or >> not. >> However, it seems potentially legit to me: >> >> In __folio_split(), when looping over folios we dereference >> `mapping` before ensuring it is non-NULL. >> >> Following code in the loop body performs such check, thus >> suggesting that `mapping` may be NULL and accessing it >> without any check may be dangerous. >> >> Add NULL check before passing it to shmem_mapping(). >> >> Cc: Zi Yan >> Fixes: 00527733d0dc ("mm/huge_memory: add two new (not yet used) functions for folio_split()") >> Addresses-Coverity-ID: 1647614 ("FORWARD_NULL") >> Signed-off-by: Antonio Quartulli >> --- >> mm/huge_memory.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 389620c65a5f..d649026db95a 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3802,7 +3802,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order, >> >> /* Some pages can be beyond EOF: drop them from cache */ >> if (new_folio->index >= end) { >> - if (shmem_mapping(mapping)) >> + if (mapping && shmem_mapping(mapping)) >> nr_shmem_dropped += folio_nr_pages(new_folio); >> else if (folio_test_clear_dirty(new_folio)) >> folio_account_cleaned( > Hi Antonio, > > Is there a way of preventing Coverity/sparse from checking certain code? > This non-NULL mapping issue pops up every time I touch the code. Probably we could make that code more readable and achieve it: diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 24aff14d22a1e..acf56aae1a2ef 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3775,6 +3775,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order, */ for (new_folio = folio_next(folio); new_folio != next_folio; new_folio = next) { + unsigned long nr_pages = folio_nr_pages(new_folio); next = folio_next(new_folio); expected_refs = folio_expected_ref_count(new_folio) + 1; @@ -3782,20 +3783,20 @@ static int __folio_split(struct folio *folio, unsigned int new_order, lru_add_split_folio(folio, new_folio, lruvec, list); - /* Some pages can be beyond EOF: drop them from cache */ - if (new_folio->index >= end) { - if (shmem_mapping(mapping)) - nr_shmem_dropped += folio_nr_pages(new_folio); - else if (folio_test_clear_dirty(new_folio)) - folio_account_cleaned( - new_folio, - inode_to_wb(mapping->host)); - __filemap_remove_folio(new_folio, NULL); - folio_put_refs(new_folio, - folio_nr_pages(new_folio)); - } else if (mapping) { - __xa_store(&mapping->i_pages, new_folio->index, - new_folio, 0); + /* Add the new folio to the page cache ... */ + if (mapping) { + /* ... however, drop folios beyond EOF. */ + if (new_folio->index >= end) { + if (shmem_mapping(mapping)) + nr_shmem_dropped += nr_pages; + else if (folio_test_clear_dirty(new_folio)) + folio_account_cleaned(new_folio, inode_to_wb(mapping->host)); + __filemap_remove_folio(new_folio, NULL); + folio_put_refs(new_folio, nr_pages); + } else { + __xa_store(&mapping->i_pages, + new_folio->index, new_folio, 0); + } } else if (swap_cache) { __xa_store(&swap_cache->i_pages, swap_cache_index(new_folio->swap), Having that logic in a helper would be cleaner, but not straight-forward due to things like nr_shmem_dropped. -- Cheers, David / dhildenb