bpf: Use memmove for bpf_dynptr_{read,write}

It may happen that destination buffer memory overlaps with memory dynptr
points to. Hence, we must use memmove to correctly copy from dynptr to
destination buffer, or source buffer to dynptr.

This actually isn't a problem right now, as memcpy implementation falls
back to memmove on detecting overlap and warns about it, but we
shouldn't be relying on that.

Acked-by: Joanne Koong <[e-mail]>
Acked-by: David Vernet <[e-mail]>
Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/helpers.c

···
 	if (err)
 		return err;
 
-	memcpy(dst, src->data + src->offset + offset, len);
+	/* Source and destination may possibly overlap, hence use memmove to
+	 * copy the data. E.g. bpf_dynptr_from_mem may create two dynptr
+	 * pointing to overlapping PTR_TO_MAP_VALUE regions.
+	 */
+	memmove(dst, src->data + src->offset + offset, len);
 
 	return 0;
 }
···
 	if (err)
 		return err;
 
-	memcpy(dst->data + dst->offset + offset, src, len);
+	/* Source and destination may possibly overlap, hence use memmove to
+	 * copy the data. E.g. bpf_dynptr_from_mem may create two dynptr
+	 * pointing to overlapping PTR_TO_MAP_VALUE regions.
+	 */
+	memmove(dst->data + dst->offset + offset, src, len);
 
 	return 0;
 }