[RFC PATCH 3/4] net: pass a kernel pointer via 'optlen_t' to proto[ops].getsockopt() hooks

Stefan Metzmacher
2025-03-31T20:12:35Z

The motivation for this is to remove the SOL_SOCKET limitation
from io_uring_cmd_getsockopt().

The reason for this limitation is that io_uring_cmd_getsockopt()
passes a kernel pointer.

The first idea would be to change the optval and optlen arguments
to the protocol specific hooks also to sockptr_t, as that
is already used for setsockopt() and also by do_sock_getsockopt()
sk_getsockopt() and BPF_CGROUP_RUN_PROG_GETSOCKOPT().

But as Linus don't like 'sockptr_t' I used a different approach.

Instead of passing the optlen as user or kernel pointer,
we only ever pass a kernel pointer and do the
translation from/to userspace in do_sock_getsockopt().

The simple solution would be to just remove the
'__user' from the int *optlen argument, but it
seems the compiler doesn't complain about
'__user' vs. without it, so instead I used
a helper struct in order to make sure everything
compiles with a typesafe change.

That together with get_optlen() and put_optlen() helper
macros make it relatively easy to review and check the
behaviour is most likely unchanged.

In order to avoid uapi changes regarding different error
code orders regarding -EFAULT, the real -EFAULT handling
is deferred to get_optlen() and put_optlen().

This allows io_uring_cmd_getsockopt() to remove the
SOL_SOCKET limitation.

Removing 'sockptr_t optlen' from existing code
is for patch for another day.

Link: https://lore.kernel.org/io-uring/86b1dce5-4bb4-4a0b-9cff-e72f488bf57d@samba.org/T/#t
Cc: Jens Axboe <axboe@kernel.dk>
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Cc: Breno Leitao <leitao@debian.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
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Cc: Andrew Lunn <andrew+netdev@lunn.ch>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
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Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: Wenjia Zhang <wenjia@linux.ibm.com>
Cc: Jan Karcher <jaka@linux.ibm.com>
Cc: "D. Wythe" <alibuda@linux.alibaba.com>
Cc: Tony Lu <tonylu@linux.alibaba.com>
Cc: Wen Gu <guwen@linux.alibaba.com>
Cc: Jon Maloy <jmaloy@redhat.com>
Cc: Boris Pismenny <borisp@nvidia.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: Martin Schiller <ms@dev.tdt.de>
Cc: "Björn Töpel" <bjorn@kernel.org>
Cc: Magnus Karlsson <magnus.karlsson@intel.com>
Cc: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
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CC: Stefan Metzmacher <metze@samba.org>
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Signed-off-by: Stefan Metzmacher <metze@samba.org>
---
include/linux/sockptr.h | 20 +++++++++++++++-----
net/socket.c | 31 +++++++++++++++++++++++++++++--
2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
index 1baf66f26f4f..06ec7fd73028 100644
--- a/include/linux/sockptr.h
+++ b/include/linux/sockptr.h
@@ -170,20 +170,25 @@ static inline int check_zeroed_sockptr(sockptr_t src, size_t offset,
}

typedef struct {
- int __user *up;
+ int *kp;
} optlen_t;

#define __check_optlen_t(__optlen) \
({ \
optlen_t *__ptr __maybe_unused = &__optlen; \
- BUILD_BUG_ON(sizeof(*((__ptr)->up)) != sizeof(int)); \
+ BUILD_BUG_ON(sizeof(*((__ptr)->kp)) != sizeof(int)); \
})

#define get_optlen(__val, __optlen) \
({ \
long __err; \
__check_optlen_t(__optlen); \
- __err = get_user(__val, __optlen.up); \
+ if ((__optlen).kp != NULL) { \
+ (__val) = *((__optlen).kp); \
+ __err = 0; \
+ } else { \
+ __err = -EFAULT; \
+ } \
__err; \
})

@@ -191,13 +196,18 @@ typedef struct {
({ \
long __err; \
__check_optlen_t(__optlen); \
- __err = put_user(__val, __optlen.up); \
+ if ((__optlen).kp != NULL) { \
+ *((__optlen).kp) = (__val); \
+ __err = 0; \
+ } else { \
+ __err = -EFAULT; \
+ } \
__err; \
})

static inline sockptr_t OPTLEN_SOCKPTR(optlen_t optlen)
{
- return (sockptr_t) { .user = optlen.up, };
+ return (sockptr_t) { .kernel = optlen.kp, .is_kernel = true };
}

#endif /* _LINUX_SOCKPTR_H */
diff --git a/net/socket.c b/net/socket.c
index fa2de12c10e6..81e5c9767bbc 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2350,15 +2350,42 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
} else if (unlikely(!ops->getsockopt)) {
err = -EOPNOTSUPP;
} else {
- optlen_t _optlen = { .up = NULL, };
+ optlen_t _optlen = { .kp = NULL, };
+ int koptlen;

if (WARN_ONCE(optval.is_kernel,
"Invalid argument type"))
return -EOPNOTSUPP;

- _optlen.up = optlen.user;
+ if (optlen.is_kernel) {
+ _optlen.kp = optlen.kernel;
+ } else if (optlen.user != NULL) {
+ /*
+ * If optlen.user is NULL,
+ * we pass _optlen.kp = NULL
+ * in order to avoid breaking
+ * any uapi for getsockopt()
+ * implementations that ignore
+ * the optlen pointer completely
+ * or do any level and optname
+ * checking before hitting a
+ * potential -EFAULT condition.
+ *
+ * Also when optlen.user is not NULL,
+ * but copy_from_sockptr() causes -EFAULT,
+ * we'll pass optlen.kp = NULL in order
+ * to defer a possible -EFAULT return
+ * to the caller to get_optlen() and put_optlen().
+ */
+ if (copy_from_sockptr(&koptlen, optlen, sizeof(koptlen)) == 0)
+ _optlen.kp = &koptlen;
+ }
err = ops->getsockopt(sock, level, optname, optval.user,
_optlen);
+ if (err != -EFAULT && _optlen.kp == &koptlen) {
+ if (copy_to_sockptr(optlen, &koptlen, sizeof(koptlen)))
+ return -EFAULT;
+ }
}

if (!compat)
--
2.34.1