linux-kernel - Re: [PATCH] proc: Bring back lseek() operations for /proc/net entries

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Message-Id: <20250822160904.6c5468bce2200cf8561970d7@linux-foundation.org>
Date: Fri, 22 Aug 2025 16:09:04 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Stefano Brivio <sbrivio@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, wangzijie
 <wangzijie1@...or.com>, Alexey Dobriyan <adobriyan@...il.com>, Christian
 Brauner <brauner@...nel.org>, passt-dev@...st.top, Al Viro
 <viro@...iv.linux.org.uk>, Ye Bin <yebin10@...wei.com>, Alexei Starovoitov
 <ast@...nel.org>, "Rick P . Edgecombe" <rick.p.edgecombe@...el.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] proc: Bring back lseek() operations for /proc/net
 entries

On Fri, 22 Aug 2025 19:23:35 +0200 Stefano Brivio <sbrivio@...hat.com> wrote:

> Commit ff7ec8dc1b64 ("proc: use the same treatment to check proc_lseek
> as ones for proc_read_iter et.al") breaks lseek() for all /proc/net
> entries, as shown for instance by pasta(1), a user-mode network
> implementation using those entries to scan for bound ports:
> 
>   $ strace -e openat,lseek -e s=none pasta -- true
>   [...]
>   openat(AT_FDCWD, "/proc/net/tcp", O_RDONLY|O_CLOEXEC) = 12
>   openat(AT_FDCWD, "/proc/net/tcp6", O_RDONLY|O_CLOEXEC) = 13
>   lseek(12, 0, SEEK_SET)                  = -1 ESPIPE (Illegal seek)
>   lseek() failed on /proc/net file: Illegal seek
>   lseek(13, 0, SEEK_SET)                  = -1 ESPIPE (Illegal seek)
>   lseek() failed on /proc/net file: Illegal seek
>   openat(AT_FDCWD, "/proc/net/udp", O_RDONLY|O_CLOEXEC) = 14
>   openat(AT_FDCWD, "/proc/net/udp6", O_RDONLY|O_CLOEXEC) = 15
>   lseek(14, 0, SEEK_SET)                  = -1 ESPIPE (Illegal seek)
>   lseek() failed on /proc/net file: Illegal seek
>   lseek(15, 0, SEEK_SET)                  = -1 ESPIPE (Illegal seek)
>   lseek() failed on /proc/net file: Illegal seek
>   [...]
> 
> That's because PROC_ENTRY_proc_lseek isn't set for /proc/net entries,
> and it's now mandatory for lseek(). In fact, flags aren't set at all
> for those entries because pde_set_flags() isn't called for them.
> 
> As commit d919b33dafb3 ("proc: faster open/read/close with "permanent"
> files") introduced flags for procfs directory entries, along with the
> pde_set_flags() helper, they weren't relevant for /proc/net entries,
> so the lack of pde_set_flags() calls in proc_create_net_*() functions
> was harmless.
> 
> Now that the calls are strictly needed for lseek() functionality,
> add them.

Thanks.  We already have
https://lkml.kernel.org/r/20250821105806.1453833-1-wangzijie1@honor.com
- does that look suitable?