Re: [PATCH] btrfs: correct the order of prelim_ref arguments in btrfs__prelim_ref

[David Sterba <dsterba@suse.cz>]

On Fri, Apr 25, 2025 at 09:25:06AM -0400, Goldwyn Rodrigues wrote:
> btrfs_prelim_ref() calls the old and new reference variables in the
> incorrect order. This causes a NULL pointer dereference because oldref
> is passed as NULL to trace_btrfs_prelim_ref_insert(). 
> 
> Note, trace_btrfs_prelim_ref_insert() is being called with newref as
> oldref (and oldref as NULL) on purpose in order to print out
> the values of newref.
> 
> To reproduce:
> echo 1 > /sys/kernel/debug/tracing/events/btrfs/btrfs_prelim_ref_insert/enable
> 
> Perform some writeback operations.
> 
> Backtrace:
> BUG: kernel NULL pointer dereference, address: 0000000000000018
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 115949067 P4D 115949067 PUD 11594a067 PMD 0 
>  Oops: Oops: 0000 [#1] SMP NOPTI
>  CPU: 1 UID: 0 PID: 1188 Comm: fsstress Not tainted 6.15.0-rc2-tester+ #47 PREEMPT(voluntary)  7ca2cef72d5e9c600f0c7718adb6462de8149622
>  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-2-gc13ff2cd-prebuilt.qemu.org 04/01/2014
>  RIP: 0010:trace_event_raw_event_btrfs__prelim_ref+0x72/0x130
>  Code: e8 43 81 9f ff 48 85 c0 74 78 4d 85 e4 0f 84 8f 00 00 00 49 8b 94 24 c0 06 00 00 48 8b 0a 48 89 48 08 48 8b 52 08 48 89 50 10 <49> 8b 55 18 48 89 50 18 49 8b 55 20 48 89 50 20 41 0f b6 55 28 88
>  RSP: 0018:ffffce44820077a0 EFLAGS: 00010286
>  RAX: ffff8c6b403f9014 RBX: ffff8c6b55825730 RCX: 304994edf9cf506b
>  RDX: d8b11eb7f0fdb699 RSI: ffff8c6b403f9010 RDI: ffff8c6b403f9010
>  RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000000000010
>  R10: 00000000ffffffff R11: 0000000000000000 R12: ffff8c6b4e8fb000
>  R13: 0000000000000000 R14: ffffce44820077a8 R15: ffff8c6b4abd1540
>  FS:  00007f4dc6813740(0000) GS:ffff8c6c1d378000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000000018 CR3: 000000010eb42000 CR4: 0000000000750ef0
>  PKRU: 55555554
>  Call Trace:
>   <TASK>
>   prelim_ref_insert+0x1c1/0x270
>   find_parent_nodes+0x12a6/0x1ee0
>   ? __entry_text_end+0x101f06/0x101f09
>   ? srso_alias_return_thunk+0x5/0xfbef5
>   ? srso_alias_return_thunk+0x5/0xfbef5
>   ? srso_alias_return_thunk+0x5/0xfbef5
>   ? srso_alias_return_thunk+0x5/0xfbef5
>   btrfs_is_data_extent_shared+0x167/0x640
>   ? fiemap_process_hole+0xd0/0x2c0
>   extent_fiemap+0xa5c/0xbc0
>   ? __entry_text_end+0x101f05/0x101f09
>   btrfs_fiemap+0x7e/0xd0
>   do_vfs_ioctl+0x425/0x9d0
>   __x64_sys_ioctl+0x75/0xc0
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

Reviewed-by: David Sterba <dsterba@suse.com>