Re: [PATCH net-next v3] rds: synchronize info callbacks with module unload

[Kuniyuki Iwashima]

From: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
Date: Sat,  1 Aug 2026 00:09:42 +0800
> rds_info_getsockopt() reads a callback from rds_info_funcs and invokes it
> without protecting the callback's lifetime.  Transport modules register
> functions stored in this array.  For example, rds_tcp.ko registers
> rds_tcp_tc_info() for RDS_INFO_TCP_SOCKETS.
> 
> This permits the following interleaving:
> 
>   CPU0                             CPU1
>   rds_info_getsockopt()
>     func = rds_tcp_tc_info
>                                    rmmod rds_tcp
>                                      rds_tcp_exit()
>                                        rds_info_deregister_func()
>                                          rds_info_funcs[offset] = NULL
>                                      free rds_tcp module text
>     func()
> 
> The reader can therefore branch to an address in unloaded module text.
> 
> Protect callback invocation with SRCU.  Enter the SRCU read-side critical
> section before loading the callback and leave it only after the callback
> returns.  Clear the callback with release semantics and call
> synchronize_srcu() before deregistration returns, preventing module unload
> from freeing its text while an old reader is still executing it.  SRCU is
> required because callbacks such as RDS_INFO_COUNTERS can sleep.
> 
> Keep the callback array unannotated and use acquire and release operations
> for publication so sparse does not have to apply __rcu through the
> function-pointer typedef.  Replace the two callback-slot BUG_ON() checks
> with WARN_ON_ONCE() and return without changing the slot on mismatch.
> 
> Link: https://lore.kernel.org/netdev/20260720184955.3008978-1-nicoyip.dev@xxxxxxxxx/
> Suggested-by: Allison Henderson <achender@xxxxxxxxxx>
> Reviewed-by: Allison Henderson <achender@xxxxxxxxxx>
> Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
> ---
> 
> Changes in v3:
> - Use ordinary loads for callback-slot validation under rds_info_lock;
>   retain acquire semantics only for the lockless reader.
> - Drop the Fixes tag because this patch targets net-next.
> - Add Allison Henderson's Reviewed-by tag.
> 
> Link: https://lore.kernel.org/netdev/20260727174826.135662-1-nicoyip.dev@xxxxxxxxx/ [v2]
> 
> Changes in v2:
> - Split the info callback/module-unload race from the independent
>   inc->i_conn lifetime bug.
> - Keep the callback array unannotated and use smp_load_acquire() and
>   smp_store_release() to resolve the sparse errors.
> - Replace the two touched callback-slot BUG_ON() checks with
>   WARN_ON_ONCE() error paths.
> - Rewrite the commit message for the callback race and omit the unrelated
>   inc->i_conn KASAN report.
> 
> Link: https://lore.kernel.org/netdev/20260720184955.3008978-1-nicoyip.dev@xxxxxxxxx/ [v1]
> 
>  net/rds/info.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/net/rds/info.c b/net/rds/info.c
> index 21b32eb16559..385fefbfa07f 100644
> --- a/net/rds/info.c
> +++ b/net/rds/info.c
> @@ -32,6 +32,7 @@
>   */
>  #include <linux/percpu.h>
>  #include <linux/seq_file.h>
> +#include <linux/srcu.h>
>  #include <linux/slab.h>
>  #include <linux/proc_fs.h>
>  #include <linux/export.h>
> @@ -68,6 +69,7 @@ struct rds_info_iterator {
>  	unsigned long offset;
>  };
>  
> +DEFINE_STATIC_SRCU(rds_info_srcu);
>  static DEFINE_SPINLOCK(rds_info_lock);
>  static rds_info_func rds_info_funcs[RDS_INFO_LAST - RDS_INFO_FIRST + 1];
>  
> @@ -78,8 +80,12 @@ void rds_info_register_func(int optname, rds_info_func func)
>  	BUG_ON(optname < RDS_INFO_FIRST || optname > RDS_INFO_LAST);
>  
>  	spin_lock(&rds_info_lock);
> -	BUG_ON(rds_info_funcs[offset]);
> -	rds_info_funcs[offset] = func;
> +	if (WARN_ON_ONCE(rds_info_funcs[offset])) {
> +		spin_unlock(&rds_info_lock);
> +		return;
> +	}
> +	/* Pair with lockless callback lookup. */
> +	smp_store_release(&rds_info_funcs[offset], func);

Why is smp_store_release() necessary here ? and same for
smp_load_acquire() on the reader.

I think just WRITE_ONCE() / READ_ONCE() should be enough.

It's just a function pointer, not struct, and the reader
does not require any partial ordering, no ?


>  	spin_unlock(&rds_info_lock);
>  }
>  EXPORT_SYMBOL_GPL(rds_info_register_func);
> @@ -91,9 +97,14 @@ void rds_info_deregister_func(int optname, rds_info_func func)
>  	BUG_ON(optname < RDS_INFO_FIRST || optname > RDS_INFO_LAST);
>  
>  	spin_lock(&rds_info_lock);
> -	BUG_ON(rds_info_funcs[offset] != func);
> -	rds_info_funcs[offset] = NULL;
> +	if (WARN_ON_ONCE(rds_info_funcs[offset] != func)) {
> +		spin_unlock(&rds_info_lock);
> +		return;
> +	}
> +	/* Hide the callback before waiting for old readers. */
> +	smp_store_release(&rds_info_funcs[offset], NULL);
>  	spin_unlock(&rds_info_lock);
> +	synchronize_srcu(&rds_info_srcu);
>  }
>  EXPORT_SYMBOL_GPL(rds_info_deregister_func);
>  
> @@ -165,6 +176,7 @@ int rds_info_getsockopt(struct socket *sock, int optname, sockopt_t *opt)
>  	int npages = 0;
>  	int ret;
>  	int len;
> +	int srcu_idx;

nit: please keep the reverse xmas tree order.


>  	int total;
>  
>  	len = opt->optlen;
> @@ -214,8 +226,11 @@ int rds_info_getsockopt(struct socket *sock, int optname, sockopt_t *opt)
>  	rdsdebug("len %d nr_pages %lu\n", len, nr_pages);
>  
>  call_func:
> -	func = rds_info_funcs[optname - RDS_INFO_FIRST];
> +	srcu_idx = srcu_read_lock(&rds_info_srcu);
> +	/* Pair with callback slot publication and removal. */
> +	func = smp_load_acquire(&rds_info_funcs[optname - RDS_INFO_FIRST]);
>  	if (!func) {
> +		srcu_read_unlock(&rds_info_srcu, srcu_idx);
>  		ret = -ENOPROTOOPT;
>  		goto out;
>  	}
> @@ -225,6 +240,7 @@ int rds_info_getsockopt(struct socket *sock, int optname, sockopt_t *opt)
>  	iter.offset = offset0;
>  
>  	func(sock, len, &iter, &lens);
> +	srcu_read_unlock(&rds_info_srcu, srcu_idx);
>  	BUG_ON(lens.each == 0);
>  
>  	total = lens.nr * lens.each;
> -- 
> 2.43.0
>