bpf: Replace __opt annotation with __nullable for kfuncs

The __opt annotation was originally introduced specifically for
buffer/size argument pairs in bpf_dynptr_slice() and
bpf_dynptr_slice_rdwr(), allowing the buffer pointer to be NULL while
still validating the size as a constant.  The __nullable annotation
serves the same purpose but is more general and is already used
throughout the BPF subsystem for raw tracepoints, struct_ops, and other
kfuncs.

This patch unifies the two annotations by replacing __opt with
__nullable.  The key change is in the verifier's
get_kfunc_ptr_arg_type() function, where mem/size pair detection is now
performed before the nullable check.  This ensures that buffer/size
pairs are correctly classified as KF_ARG_PTR_TO_MEM_SIZE even when the
buffer is nullable, while adding an !arg_mem_size condition to the
nullable check prevents interference with mem/size pair handling.

When processing KF_ARG_PTR_TO_MEM_SIZE arguments, the verifier now uses
is_kfunc_arg_nullable() instead of the removed is_kfunc_arg_optional()
to determine whether to skip size validation for NULL buffers.

This is the first documentation added for the __nullable annotation,
which has been in use since it was introduced but was previously
undocumented.

No functional changes to verifier behavior - nullable buffer/size pairs
continue to work exactly as before.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102221513.1961781-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4e9667ed6630..a63e47d2109c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1434,7 +1434,7 @@ bool __bpf_dynptr_is_rdonly(const struct bpf_dynptr_kern *ptr);
 int __bpf_dynptr_write(const struct bpf_dynptr_kern *dst, u64 offset,
 		       void *src, u64 len, u64 flags);
 void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr *p, u64 offset,
-			    void *buffer__opt, u64 buffer__szk);
+			    void *buffer__nullable, u64 buffer__szk);
 
 static inline int bpf_dynptr_check_off_len(const struct bpf_dynptr_kern *ptr, u64 offset, u64 len)
 {