Re: Regression: NULL pointer dereference after NFS_V4_2_READ_PLUS (commit 7fd461c47)

[Krzysztof Kozlowski]

On 07/01/2023 16:44, Krzysztof Kozlowski wrote:
> Hi,
> 
> Bisect identified commit 7fd461c47c6c ("NFSv4.2: Change the default
> KConfig value for READ_PLUS") as one leading to NULL pointer exception
> when mounting NFS root on NFSv4 client:
> 
> [   25.739003] systemd[1]: Set hostname to <odroidhc1>.
> [   25.771714] systemd[1]: Failed to bump fs.file-max, ignoring: Invalid
> argument
> [   26.199478] 8<--- cut here ---
> [   26.201366] Unable to handle kernel NULL pointer dereference at
> virtual address 00000004
> ...
> [   26.555522]  mmiocpy from xdr_inline_decode+0xec/0x16c
> [   26.560628]  xdr_inline_decode from nfs4_xdr_dec_read_plus+0x178/0x358
> [   26.567130]  nfs4_xdr_dec_read_plus from call_decode+0x204/0x304
> 
> Full OOPS attached. Full log available here:
> https://krzk.eu/#/builders/21/builds/3901/steps/15/logs/serial0
> 
> Disabling NFS_V4_2_READ_PLUS fixes the issue, so obviously the commit is
> not the cause, but rather making it default caused the regression.
> 
> I did not make the bisect yet which commit introduced it, if every
> config includes NFS_V4_2_READ_PLUS.

When every kernel is built with NFS_V4_2_READ_PLUS, bisect pointed to:
d3b00a802c84 ("NFS: Replace the READ_PLUS decoding code")

commit d3b00a802c845a6021148ce2e669b5a0b5729959
Author: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
Date:   Thu Jul 21 14:21:34 2022 -0400

    NFS: Replace the READ_PLUS decoding code

    We now take a 2-step process that allows us to place data and hole
    segments directly at their final position in the xdr_stream without
    needing to do a bunch of redundant copies to expand holes. Due to the
    variable lengths of each segment, the xdr metadata might cross page
    boundaries which I account for by setting a small scratch buffer so
    xdr_inline_decode() won't fail.

    Signed-off-by: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
    Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>

With a trace:
[   25.898462] systemd[1]: Set hostname to <odroidhc1>.
[   25.933746] systemd[1]: Failed to bump fs.file-max, ignoring: Invalid
argument
[   25.986237] random: crng init done
[   26.264564] 8<--- cut here ---
[   26.266823] Unable to handle kernel NULL pointer dereference at
virtual address 00000fe8
...
[   26.597263]  nfs4_xdr_dec_read_plus from call_decode+0x204/0x304
[   26.603222]  call_decode from __rpc_execute+0xd0/0x890
[   26.608328]  __rpc_execute from rpc_async_schedule+0x1c/0x34
[   26.613960]  rpc_async_schedule from process_one_work+0x294/0x790
[   26.620030]  process_one_work from worker_thread+0x54/0x518
[   26.625570]  worker_thread from kthread+0xf4/0x128
[   26.630336]  kthread from ret_from_fork+0x14/0x2c


Best regards,
Krzysztof