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

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Message-ID: <20250822172335.3187858-1-sbrivio@redhat.com>
Date: Fri, 22 Aug 2025 19:23:35 +0200
From: Stefano Brivio <sbrivio@...hat.com>
To: linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.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: [PATCH] proc: Bring back lseek() operations for /proc/net entries

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@...hat.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