Re: [PATCH v1 1/1] SUNRPC: Make enough room in servername[] for AF_UNIX addresses

[Mirsad Todorovac <mtodorovac69@gmail.com>]

On 9/24/24 12:43, NeilBrown wrote:
> On Tue, 24 Sep 2024, Mirsad Todorovac wrote:
>> Hi, Neil,
>>
>> Apparently I was duplicating work.
>>
>> However, using
>>
>> 	char servername[UNIX_PATH_MAX];
>>
>> has some advantages when compared to hard-coded integer?
>>
>> Correct me if I'm wrong.
> 
> I think you are wrong.  I agree that 48 is a poor choice.  I think that
> UNIX_PATH_MAX is a poor choice too.  The "servername" string is used for
> things other than a UNIX socket path.
> Did you read all of the thread that I provided a link for?  I suggest a
> more meaningful number in one of the later messages.

I see. Thanks for the tip. However, if UNIX_PATH_MAX ever changes in the
future, the decl

    char servername[108];

might be missed when fixing all the changes caused by the change of the
macro definition? Am I wrong again?

Making it logically depend on the system limits might save some headache
in the future, perhaps.

If really the biggest string that will be copied there is: "/var/run/rpcbind.sock",
you are then right - stack space is precious commodity, and allocating
via kmalloc() might preempt the caller thread.

However, you got to this five weeks earlier - but the patch did not
propagate to the main vanilla torvalds tree.

Best regards,
Mirsad Todorovac

> But I really think that the problem here is the warning.  The servername
> array is ALWAYS big enough for any string that will actually be copied
> into it.  gcc isn't clever enough to detect that, but it tries to be
> clever enough to tell you that even though you used snprintf so you know
> that the string might in theory overflow, it decides to warn you about
> something you already know.
> 
> i.e.  if you want to fix this, I would rather you complain the the
> compiler writers.  Or maybe suggest a #pragma to silence the warning in
> this case.  Or maybe examine all of the code instead of the one line
> that triggers the warning and see if there is a better approach to
> providing the functionality that is being provided here.
> 
> NeilBrown
> 
>>
>> Best regards,
>> Mirsad Todorovac
>>
>> On 9/23/24 23:24, NeilBrown wrote:
>>> On Tue, 24 Sep 2024, Mirsad Todorovac wrote:
>>>> GCC 13.2.0 reported with W=1 build option the following warning:
>>>
>>> See
>>>   https://lore.kernel.org/all/20240814093853.48657-1-kunwu.chan@linux.dev/
>>>
>>> I don't think anyone really cares about this one.
>>>
>>> NeilBrown
>>>
>>>
>>>>
>>>> net/sunrpc/clnt.c: In function ‘rpc_create’:
>>>> net/sunrpc/clnt.c:582:75: warning: ‘%s’ directive output may be truncated writing up to 107 bytes into \
>>>> 					a region of size 48 [-Wformat-truncation=]
>>>>   582 |                                 snprintf(servername, sizeof(servername), "%s",
>>>>       |                                                                           ^~
>>>> net/sunrpc/clnt.c:582:33: note: ‘snprintf’ output between 1 and 108 bytes into a destination of size 48
>>>>   582 |                                 snprintf(servername, sizeof(servername), "%s",
>>>>       |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>   583 |                                          sun->sun_path);
>>>>       |                                          ~~~~~~~~~~~~~~
>>>>
>>>>    548         };
>>>>  → 549         char servername[48];
>>>>    550         struct rpc_clnt *clnt;
>>>>    551         int i;
>>>>    552
>>>>    553         if (args->bc_xprt) {
>>>>    554                 WARN_ON_ONCE(!(args->protocol & XPRT_TRANSPORT_BC));
>>>>    555                 xprt = args->bc_xprt->xpt_bc_xprt;
>>>>    556                 if (xprt) {
>>>>    557                         xprt_get(xprt);
>>>>    558                         return rpc_create_xprt(args, xprt);
>>>>    559                 }
>>>>    560         }
>>>>    561
>>>>    562         if (args->flags & RPC_CLNT_CREATE_INFINITE_SLOTS)
>>>>    563                 xprtargs.flags |= XPRT_CREATE_INFINITE_SLOTS;
>>>>    564         if (args->flags & RPC_CLNT_CREATE_NO_IDLE_TIMEOUT)
>>>>    565                 xprtargs.flags |= XPRT_CREATE_NO_IDLE_TIMEOUT;
>>>>    566         /*
>>>>    567          * If the caller chooses not to specify a hostname, whip
>>>>    568          * up a string representation of the passed-in address.
>>>>    569          */
>>>>    570         if (xprtargs.servername == NULL) {
>>>>    571                 struct sockaddr_un *sun =
>>>>    572                                 (struct sockaddr_un *)args->address;
>>>>    573                 struct sockaddr_in *sin =
>>>>    574                                 (struct sockaddr_in *)args->address;
>>>>    575                 struct sockaddr_in6 *sin6 =
>>>>    576                                 (struct sockaddr_in6 *)args->address;
>>>>    577
>>>>    578                 servername[0] = '\0';
>>>>    579                 switch (args->address->sa_family) {
>>>>  → 580                 case AF_LOCAL:
>>>>  → 581                         if (sun->sun_path[0])
>>>>  → 582                                 snprintf(servername, sizeof(servername), "%s",
>>>>  → 583                                          sun->sun_path);
>>>>  → 584                         else
>>>>  → 585                                 snprintf(servername, sizeof(servername), "@%s",
>>>>  → 586                                          sun->sun_path+1);
>>>>  → 587                         break;
>>>>    588                 case AF_INET:
>>>>    589                         snprintf(servername, sizeof(servername), "%pI4",
>>>>    590                                  &sin->sin_addr.s_addr);
>>>>    591                         break;
>>>>    592                 case AF_INET6:
>>>>    593                         snprintf(servername, sizeof(servername), "%pI6",
>>>>    594                                  &sin6->sin6_addr);
>>>>    595                         break;
>>>>    596                 default:
>>>>    597                         /* caller wants default server name, but
>>>>    598                          * address family isn't recognized. */
>>>>    599                         return ERR_PTR(-EINVAL);
>>>>    600                 }
>>>>    601                 xprtargs.servername = servername;
>>>>    602         }
>>>>    603
>>>>    604         xprt = xprt_create_transport(&xprtargs);
>>>>    605         if (IS_ERR(xprt))
>>>>    606                 return (struct rpc_clnt *)xprt;
>>>>
>>>> After the address family AF_LOCAL was added in the commit 176e21ee2ec89, the old hard-coded
>>>> size for servername of char servername[48] no longer fits. The maximum AF_UNIX address size
>>>> has now grown to UNIX_PATH_MAX defined as 108 in "include/uapi/linux/un.h" .
>>>>
>>>> The lines 580-587 were added later, addressing the leading zero byte '\0', but did not fix
>>>> the hard-coded servername limit.
>>>>
>>>> The AF_UNIX address was truncated to 47 bytes + terminating null byte. This patch will fix the
>>>> servername in AF_UNIX family to the maximum permitted by the system:
>>>>
>>>>    548         };
>>>>  → 549         char servername[UNIX_PATH_MAX];
>>>>    550         struct rpc_clnt *clnt;
>>>>
>>>> Fixes: 4388ce05fa38b ("SUNRPC: support abstract unix socket addresses")
>>>> Fixes: 510deb0d7035d ("SUNRPC: rpc_create() default hostname should support AF_INET6 addresses")
>>>> Fixes: 176e21ee2ec89 ("SUNRPC: Support for RPC over AF_LOCAL transports")
>>>> Cc: Neil Brown <neilb@suse.de>
>>>> Cc: Chuck Lever <chuck.lever@oracle.com>
>>>> Cc: Trond Myklebust <trondmy@kernel.org>
>>>> Cc: Anna Schumaker <anna@kernel.org>
>>>> Cc: Jeff Layton <jlayton@kernel.org>
>>>> Cc: Olga Kornievskaia <okorniev@redhat.com>
>>>> Cc: Dai Ngo <Dai.Ngo@oracle.com>
>>>> Cc: Tom Talpey <tom@talpey.com>
>>>> Cc: "David S. Miller" <davem@davemloft.net>
>>>> Cc: Eric Dumazet <edumazet@google.com>
>>>> Cc: Jakub Kicinski <kuba@kernel.org>
>>>> Cc: Paolo Abeni <pabeni@redhat.com>
>>>> Cc: linux-nfs@vger.kernel.org
>>>> Cc: netdev@vger.kernel.org
>>>> Cc: linux-kernel@vger.kernel.org
>>>> Signed-off-by: Mirsad Todorovac <mtodorovac69@gmail.com>
>>>> ---
>>>>  v1:
>>>> 	initial version.
>>>>
>>>>  net/sunrpc/clnt.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
>>>> index 09f29a95f2bc..67099719893e 100644
>>>> --- a/net/sunrpc/clnt.c
>>>> +++ b/net/sunrpc/clnt.c
>>>> @@ -546,7 +546,7 @@ struct rpc_clnt *rpc_create(struct rpc_create_args *args)
>>>>  		.connect_timeout = args->connect_timeout,
>>>>  		.reconnect_timeout = args->reconnect_timeout,
>>>>  	};
>>>> -	char servername[48];
>>>> +	char servername[UNIX_PATH_MAX];
>>>>  	struct rpc_clnt *clnt;
>>>>  	int i;
>>>>  
>>>> -- 
>>>> 2.43.0
>>>>
>>>>
>>>
>>
>