llext: stricter alignment checks for MPU/MMU regions

This patch moves the alignment checks for MPU and MMU regions to the
beginning of the llext_copy_region() function. This is done to ensure
that the correct region alignment and size are used even in the case
where the region is reused from the ELF file buffer, avoiding MMU/MPU
configuration issues.

This also relaxes the same checks for regions that are accessed by the
kernel only (e.g. symbol and string tables), which do not need special
MMU/MPU treatment.

Author: pillo79

subsys/llext/llext_mem.c

@@ -71,6 +71,29 @@ static int llext_copy_region(struct llext_loader *ldr, struct llext *ext,
 	}
 	ext->mem_size[mem_idx] = region_alloc;
 
+	/*
+	 * Calculate the minium region size and alignment that can satisfy
+	 * MMU/MPU requirements. This only applies to regions that contain
+	 * program-accessible data (not to string tables, for example).
+	 */
+	if (region->sh_flags & SHF_ALLOC) {
+		if (IS_ENABLED(CONFIG_ARM_MPU)) {
+			/* On ARM with an MPU, regions must be sized and
+			 * aligned to the same power of two (larger than 32).
+			 */
+			uintptr_t block_sz = MAX(MAX(region_alloc, region_align), LLEXT_PAGE_SIZE);
+
+			block_sz = 1 << LOG2CEIL(block_sz); /* align to next power of two */
+			region_alloc = block_sz;
+			region_align = block_sz;
+		} else if (IS_ENABLED(CONFIG_MMU)) {
+			/* On MMU targets, round the region to multiples of
+			 * LLEXT_PAGE_SIZE. */
+			region_alloc = ROUND_UP(region_alloc, LLEXT_PAGE_SIZE);
+			region_align = MAX(region_align, LLEXT_PAGE_SIZE);
+		}
+	}
+
 	if (ldr->storage == LLEXT_STORAGE_WRITABLE) {
 		/*
 		 * Try to reuse data areas from the ELF buffer, if possible.
@@ -114,24 +137,7 @@ static int llext_copy_region(struct llext_loader *ldr, struct llext *ext,
 		return -EFAULT;
 	}
 
-	/*
-	 * Calculate the desired region size and alignment for a new allocation.
-	 */
-	if (IS_ENABLED(CONFIG_ARM_MPU)) {
-		/* On ARM with an MPU, regions must be sized and aligned to the same
-		 * power of two (larger than 32).
-		 */
-		uintptr_t block_size = MAX(MAX(region_alloc, region_align), LLEXT_PAGE_SIZE);
-
-		block_size = 1 << LOG2CEIL(block_size); /* align to next power of two */
-		region_alloc = block_size;
-		region_align = block_size;
-	} else {
-		/* Otherwise, round the region to multiples of LLEXT_PAGE_SIZE. */
-		region_alloc = ROUND_UP(region_alloc, LLEXT_PAGE_SIZE);
-		region_align = MAX(region_align, LLEXT_PAGE_SIZE);
-	}
-
+	/* Allocate a suitably aligned area for the region. */
 	ext->mem[mem_idx] = llext_aligned_alloc(region_align, region_alloc);
 	if (!ext->mem[mem_idx]) {
 		LOG_ERR("Failed allocating %zd bytes %zd-aligned for region %d",