llext: add llext_peek()

The only way so far to access extension images is via a memory
buffer. Since this, supposedly, will also be a rather common method,
it makes sense to add a method to access extension data quickly by
obtaining a pointer instead of copying data into local buffers. Add a
llext_peek() method for that.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

include/zephyr/llext/buf_loader.h

@@ -37,6 +37,7 @@ struct llext_buf_loader {
 /** @cond ignore */
 int llext_buf_read(struct llext_loader *ldr, void *buf, size_t len);
 int llext_buf_seek(struct llext_loader *ldr, size_t pos);
+void *llext_buf_peek(struct llext_loader *ldr, size_t pos);
 /** @endcond */
 
 /**
@@ -49,7 +50,8 @@ int llext_buf_seek(struct llext_loader *ldr, size_t pos);
 	{						\
 		.loader = {				\
 			.read = llext_buf_read,		\
-			.seek = llext_buf_seek		\
+			.seek = llext_buf_seek,		\
+			.peek = llext_buf_peek,		\
 		},					\
 		.buf = (_buf),				\
 		.len = (_buf_len),			\

include/zephyr/llext/loader.h

@@ -73,7 +73,19 @@ struct llext_loader {
 	 * @retval 0 Success
 	 * @retval -errno Error reading (any errno)
 	 */
-	int (*seek)(struct llext_loader *s, size_t pos);
+	int (*seek)(struct llext_loader *ldr, size_t pos);
+
+	/**
+	 * @brief Peek at an absolute location
+	 *
+	 * Return a pointer to the buffer at specified offset.
+	 *
+	 * @param[in] ldr Loader
+	 * @param[in] pos Position to obtain a pointer to
+	 *
+	 * @retval pointer into the buffer
+	 */
+	void *(*peek)(struct llext_loader *ldr, size_t pos);
 
 	/** @cond ignore */
 	elf_ehdr_t hdr;

subsys/llext/buf_loader.c

@@ -29,3 +29,10 @@ int llext_buf_seek(struct llext_loader *l, size_t pos)
 
 	return 0;
 }
+
+void *llext_buf_peek(struct llext_loader *l, size_t pos)
+{
+	struct llext_buf_loader *buf_l = CONTAINER_OF(l, struct llext_buf_loader, loader);
+
+	return (void *)(buf_l->buf + pos);
+}

subsys/llext/llext.c

@@ -31,6 +31,15 @@ static inline int llext_seek(struct llext_loader *l, size_t pos)
 	return l->seek(l, pos);
 }
 
+static inline void *llext_peek(struct llext_loader *l, size_t pos)
+{
+	if (l->peek) {
+		return l->peek(l, pos);
+	}
+
+	return NULL;
+}
+
 static sys_slist_t _llext_list = SYS_SLIST_STATIC_INIT(&_llext_list);
 
 sys_slist_t *llext_list(void)