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

[Stefano Brivio <sbrivio@redhat.com>]

Hi wangzijie,

On Sat, 23 Aug 2025 09:53:49 +0800
wangzijie <wangzijie1@honor.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.
> > 
> > Fixes: ff7ec8dc1b64 ("proc: use the same treatment to check proc_lseek as ones for proc_read_iter et.al")
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > ---
> >  fs/proc/generic.c  | 2 +-
> >  fs/proc/internal.h | 1 +
> >  fs/proc/proc_net.c | 4 ++++
> >  3 files changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/proc/generic.c b/fs/proc/generic.c
> > index 76e800e38c8f..57ec5e385d1b 100644
> > --- a/fs/proc/generic.c
> > +++ b/fs/proc/generic.c
> > @@ -561,7 +561,7 @@ struct proc_dir_entry *proc_create_reg(const char *name, umode_t mode,
> >  	return p;
> >  }
> >  
> > -static void pde_set_flags(struct proc_dir_entry *pde)
> > +void pde_set_flags(struct proc_dir_entry *pde)
> >  {
> >  	if (pde->proc_ops->proc_flags & PROC_ENTRY_PERMANENT)
> >  		pde->flags |= PROC_ENTRY_PERMANENT;
> > diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> > index e737401d7383..a358974f14d2 100644
> > --- a/fs/proc/internal.h
> > +++ b/fs/proc/internal.h
> > @@ -284,6 +284,7 @@ extern struct dentry *proc_lookup(struct inode *, struct dentry *, unsigned int)
> >  struct dentry *proc_lookup_de(struct inode *, struct dentry *, struct proc_dir_entry *);
> >  extern int proc_readdir(struct file *, struct dir_context *);
> >  int proc_readdir_de(struct file *, struct dir_context *, struct proc_dir_entry *);
> > +void pde_set_flags(struct proc_dir_entry *pde);
> >  
> >  static inline void pde_get(struct proc_dir_entry *pde)
> >  {
> > diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
> > index 52f0b75cbce2..20bc7481b02c 100644
> > --- a/fs/proc/proc_net.c
> > +++ b/fs/proc/proc_net.c
> > @@ -124,6 +124,7 @@ struct proc_dir_entry *proc_create_net_data(const char *name, umode_t mode,
> >  	p->proc_ops = &proc_net_seq_ops;
> >  	p->seq_ops = ops;
> >  	p->state_size = state_size;
> > +	pde_set_flags(p);
> >  	return proc_register(parent, p);
> >  }
> >  EXPORT_SYMBOL_GPL(proc_create_net_data);
> > @@ -170,6 +171,7 @@ struct proc_dir_entry *proc_create_net_data_write(const char *name, umode_t mode
> >  	p->seq_ops = ops;
> >  	p->state_size = state_size;
> >  	p->write = write;
> > +	pde_set_flags(p);
> >  	return proc_register(parent, p);
> >  }
> >  EXPORT_SYMBOL_GPL(proc_create_net_data_write);
> > @@ -217,6 +219,7 @@ struct proc_dir_entry *proc_create_net_single(const char *name, umode_t mode,
> >  	pde_force_lookup(p);
> >  	p->proc_ops = &proc_net_single_ops;
> >  	p->single_show = show;
> > +	pde_set_flags(p);
> >  	return proc_register(parent, p);
> >  }
> >  EXPORT_SYMBOL_GPL(proc_create_net_single);
> > @@ -261,6 +264,7 @@ struct proc_dir_entry *proc_create_net_single_write(const char *name, umode_t mo
> >  	p->proc_ops = &proc_net_single_ops;
> >  	p->single_show = show;
> >  	p->write = write;
> > +	pde_set_flags(p);
> >  	return proc_register(parent, p);
> >  }
> >  EXPORT_SYMBOL_GPL(proc_create_net_single_write);
> > -- 
> > 2.43.0  
> 
> Hi Stefano,
> Thanks for your patch, Lars reported this bug last week:
> https://lore.kernel.org/all/20250815195616.64497967@chagall.paradoxon.rec/

Apologies, I didn't see that. It's definitely the same issue.

> Jiri suggested to make pde_set_flags() part of proc_register(). I think it can help
> to avoid lack of pde_set_flags() calls in the future and make code clean.

Right, I was pondering to try something like that, but I just wanted to
submit a minimal fix for the moment being.

In any case, you have it ready, and it's obviously cleaner than my
solution, so, of course, let's discard my patch.

> I have submitted a patch:
> https://lore.kernel.org/all/20250821105806.1453833-1-wangzijie1@honor.com

I just tested it, it works for me. Thanks for fixing that!

-- 
Stefano