When checking whether the edges of adjacent subrequests touch, the `prev` variable is deferenced, but it might not have been initialized. This causes crashes like this one:

BUG: unable to handle page fault for address: 0000000181343843 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 8000001c66db0067 P4D 8000001c66db0067 PUD 0 Oops: Oops: 0000 [#1] SMP PTI CPU: 1 UID: 33333 PID: 24424 Comm: php-cgi8.2 Kdump: loaded Not tainted 6.13.2-cm4all0-hp+ #427 Hardware name: HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 11/23/2021 RIP: 0010:netfs_consume_read_data.isra.0+0x5ef/0xb00 Code: fe ff ff 48 8b 83 88 00 00 00 48 8b 4c 24 30 4c 8b 43 78 48 85 c0 48 8d 51 70 75 20 48 8b 73 30 48 39 d6 74 17 48 8b 7c 24 40 <48> 8b 4f 78 48 03 4f 68 48 39 4b 68 0f 84 ab 02 00 00 49 29 c0 48 RSP: 0000:ffffc90037adbd00 EFLAGS: 00010283 RAX: 0000000000000000 RBX: ffff88811bda0600 RCX: ffff888620e7b980 RDX: ffff888620e7b9f0 RSI: ffff88811bda0428 RDI: 00000001813437cb RBP: 0000000000000000 R08: 0000000000004000 R09: 0000000000000000 R10: ffffffff82e070c0 R11: 0000000007ffffff R12: 0000000000004000 R13: ffff888620e7bb68 R14: 0000000000008000 R15: ffff888620e7bb68 FS: 00007ff2e0e7ddc0(0000) GS:ffff88981f840000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000181343843 CR3: 0000001bc10ba006 CR4: 00000000001706f0 Call Trace: <TASK> ? __die+0x1f/0x60 ? page_fault_oops+0x15c/0x450 ? search_extable+0x22/0x30 ? netfs_consume_read_data.isra.0+0x5ef/0xb00 ? search_module_extables+0xe/0x40 ? exc_page_fault+0x5e/0x100 ? asm_exc_page_fault+0x22/0x30 ? netfs_consume_read_data.isra.0+0x5ef/0xb00 ? intel_iommu_unmap_pages+0xaa/0x190 ? __pfx_cachefiles_read_complete+0x10/0x10 netfs_read_subreq_terminated+0x24f/0x390 cachefiles_read_complete+0x48/0xf0 iomap_dio_bio_end_io+0x125/0x160 blk_update_request+0xea/0x3e0 scsi_end_request+0x27/0x190 scsi_io_completion+0x43/0x6c0 blk_complete_reqs+0x40/0x50 handle_softirqs+0xd1/0x280 irq_exit_rcu+0x91/0xb0 common_interrupt+0x3b/0xa0 asm_common_interrupt+0x22/0x40 RIP: 0033:0x55fe8470d2ab Code: 00 00 3c 7e 74 3b 3c b6 0f 84 dd 03 00 00 3c 1e 74 2f 83 c1 01 48 83 c2 38 48 83 c7 30 44 39 d1 74 3e 48 63 42 08 85 c0 79 a3 <49> 8b 46 48 8b 04 38 f6 c4 04 75 0b 0f b6 42 30 83 e0 0c 3c 04 75 RSP: 002b:00007ffca5ef2720 EFLAGS: 00000216 RAX: 0000000000000023 RBX: 0000000000000008 RCX: 000000000000001b RDX: 00007ff2e0cdb6f8 RSI: 0000000000000006 RDI: 0000000000000510 RBP: 00007ffca5ef27a0 R08: 00007ffca5ef2720 R09: 0000000000000001 R10: 000000000000001e R11: 00007ff2e0c10d08 R12: 0000000000000001 R13: 0000000000000120 R14: 00007ff2e0cb1ed0 R15: 00000000000000b0 </TASK>

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading") Cc: stable@vger.kernel.org Signed-off-by: Max Kellermann max.kellermann@ionos.com --- David/Greg: just like the other two netfs patches I sent yesterday, this one doesn't apply to v6.14 as it was obsoleted by commit e2d46f2ec332 ("netfs: Change the read result collector to only use one work item").

v1->v2: duplicate "prev" assigment removed.

Signed-off-by: Max Kellermann max.kellermann@ionos.com --- fs/netfs/read_collect.c | 21 +++++++++++---------- 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/fs/netfs/read_collect.c b/fs/netfs/read_collect.c index e8624f5c7fcc..8878b46589ff 100644 --- a/fs/netfs/read_collect.c +++ b/fs/netfs/read_collect.c @@ -258,17 +258,18 @@ static bool netfs_consume_read_data(struct netfs_io_subrequest *subreq, bool was */ if (!subreq->consumed && !prev_donated && - !list_is_first(&subreq->rreq_link, &rreq->subrequests) && - subreq->start == prev->start + prev->len) { + !list_is_first(&subreq->rreq_link, &rreq->subrequests)) { prev = list_prev_entry(subreq, rreq_link); - WRITE_ONCE(prev->next_donated, prev->next_donated + subreq->len); - subreq->start += subreq->len; - subreq->len = 0; - subreq->transferred = 0; - trace_netfs_donate(rreq, subreq, prev, subreq->len, - netfs_trace_donate_to_prev); - trace_netfs_sreq(subreq, netfs_sreq_trace_donate_to_prev); - goto remove_subreq_locked; + if (subreq->start == prev->start + prev->len) { + WRITE_ONCE(prev->next_donated, prev->next_donated + subreq->len); + subreq->start += subreq->len; + subreq->len = 0; + subreq->transferred = 0; + trace_netfs_donate(rreq, subreq, prev, subreq->len, + netfs_trace_donate_to_prev); + trace_netfs_sreq(subreq, netfs_sreq_trace_donate_to_prev); + goto remove_subreq_locked; + } }

/* If we can't donate down the chain, donate up the chain instead. */