llext: optimize allocations for read-only sections

When loading an extension, read-only sections that do not have
relocations and whose data is accessible from the ELF buffer can be
directly mapped as-is in the extension memory. This avoids the need to
allocate and copy unmodified data from the ELF buffer to the LLEXT heap.

Signed-off-by: Luca Burelli <[email protected]>

Author: pillo79

subsys/llext/llext_load.c

@@ -216,6 +216,15 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext,
 	int i, j;
 	const char *name;
 
+	/* early loop to mark sections where relocations will be applied */
+	for (i = 0; i < ldr->sect_cnt; ++i) {
+		elf_shdr_t *shdr = ldr->sect_hdrs + i;
+
+		if (shdr->sh_type == SHT_REL || shdr->sh_type == SHT_RELA) {
+			ldr->sect_map[shdr->sh_info].has_relocs = true;
+		}
+	}
+
 	for (i = 0; i < ldr->sect_cnt; ++i) {
 		elf_shdr_t *shdr = ldr->sect_hdrs + i;
 
@@ -301,6 +310,7 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext,
 			 * region descriptor.
 			 */
 			memcpy(region, shdr, sizeof(*region));
+			region->sh_info = 0; /* reuse for relocation status */
 		} else {
 			/* Make sure this section is compatible with the region */
 			if ((shdr->sh_flags & SHF_BASIC_TYPE_MASK) !=
@@ -360,6 +370,9 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext,
 			region->sh_offset = bot_ofs;
 			region->sh_size = top_ofs - bot_ofs;
 		}
+
+		/* Update this region's relocation status */
+		region->sh_info |= ldr->sect_map[i].has_relocs;
 	}
 
 	/*

subsys/llext/llext_mem.c

@@ -69,9 +69,13 @@ static int llext_copy_region(struct llext_loader *ldr, struct llext *ext,
 	}
 	ext->mem_size[mem_idx] = region->sh_size;
 
-	if (region->sh_type != SHT_NOBITS &&
-	    !IS_ENABLED(CONFIG_LLEXT_HW_MEMORY_CHECKS) &&
-	    ldr->storage == LLEXT_STORAGE_WRITABLE) {
+	if (region->sh_type != SHT_NOBITS &&               /* region has data in the ELF  */
+	    !IS_ENABLED(CONFIG_LLEXT_HW_MEMORY_CHECKS) &&  /* && no mem protection rules  */
+	    (ldr->storage == LLEXT_STORAGE_WRITABLE ||     /* && writable storage         */
+	     (ldr->storage == LLEXT_STORAGE_PERSISTENT &&  /*    || persistent storage    */
+	      !(region->sh_flags & SHF_WRITE) &&           /*       && read-only region   */
+	      !region->sh_info))                           /*       && no relocs to apply */
+	   ) {
 		/* Directly use data from the ELF buffer */
 		ext->mem[mem_idx] = llext_peek(ldr, region->sh_offset);
 		if (ext->mem[mem_idx]) {

subsys/llext/llext_priv.h

@@ -13,6 +13,7 @@
 struct llext_elf_sect_map {
 	enum llext_mem mem_idx;
 	size_t offset;
+	bool has_relocs;
 };
 
 /*