From f4f373d65de6d665a3d96222ae1c3ad7b84ca4c2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mario=20H=C3=BCttel?= <mario.huettel@gmx.net>
Date: Sat, 17 Dec 2022 19:33:13 +0100
Subject: [PATCH] Readout elf endianess and print error on mismatch

---
 include/patchelfcrc/elfpatch.h |  2 +-
 src/elfpatch.c                 | 28 +++++++++++++++++++++++++---
 src/main.c                     |  2 +-
 3 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/include/patchelfcrc/elfpatch.h b/include/patchelfcrc/elfpatch.h
index c8646db..c16faac 100644
--- a/include/patchelfcrc/elfpatch.h
+++ b/include/patchelfcrc/elfpatch.h
@@ -41,7 +41,7 @@ enum crc_format {
     FORMAT_STRUCT,
 };
 
-elfpatch_handle_t *elf_patch_open(const char *path, bool readonly);
+elfpatch_handle_t *elf_patch_open(const char *path, bool readonly, bool expect_little_endian);
 
 /**
  * @brief Check if a section is present in file
diff --git a/src/elfpatch.c b/src/elfpatch.c
index b2cb876..e1f7ef1 100644
--- a/src/elfpatch.c
+++ b/src/elfpatch.c
@@ -293,9 +293,10 @@ static int elf_patch_update_info(elfpatch_handle_t *ep)
 	return 0;
 }
 
-elfpatch_handle_t *elf_patch_open(const char *path, bool readonly)
+elfpatch_handle_t *elf_patch_open(const char *path, bool readonly, bool expect_little_endian)
 {
 	struct elfpatch *ep;
+	const char *ident;
 
 	/* This is important to guarantee structure packing behavior */
 	CRC_OUT_CHECK_STRUCT_SIZES;
@@ -320,7 +321,7 @@ elfpatch_handle_t *elf_patch_open(const char *path, bool readonly)
 		goto close_fd;
 	}
 
-	/* Prewvent Libelf from relayouting the sections, which would brick the load segments */
+	/* Prevent Libelf from relayouting the sections, which would brick the load segments */
 	elf_flagelf(ep->elf, ELF_C_SET, ELF_F_LAYOUT);
 
 	if (elf_patch_update_info(ep)) {
@@ -328,6 +329,27 @@ elfpatch_handle_t *elf_patch_open(const char *path, bool readonly)
 		goto close_elf;
 	}
 
+	ident = elf_getident(ep->elf, NULL);
+	if (ident) {
+		switch (ident[5]) {
+		case 1:
+			print_debug("ELF Endianess: little\n");
+			if (!expect_little_endian) {
+				print_err("Big endian format expected. File is little endian. Double check settings!\n");
+			}
+			break;
+		case 2:
+			print_debug("ELF Endianess: big\n");
+			if (expect_little_endian) {
+				print_err("Little endian format expected. File is big endian. Double check settings!\n");
+			}
+			break;
+		default:
+			print_err("Cannot determine endianess of ELF file. EI_DATA is: %d\n", ident[5]);
+			break;
+		}
+	}
+
 	return (elfpatch_handle_t *)ep;
 close_elf:
 	if (ep->elf) {
@@ -634,7 +656,7 @@ int elf_patch_write_crcs_to_section(elfpatch_handle_t *ep, const char *section,
 	}
 
 	/* Flag section data as invalid to trigger rewrite.
-	 * This is needed to to the forced memory layout
+	 * This is needed due to the forced memory layout
 	 */
 	elf_flagdata(output_sec_data, ELF_C_SET, ELF_F_DIRTY);
 	ret = 0;
diff --git a/src/main.c b/src/main.c
index a81fb34..97c2047 100644
--- a/src/main.c
+++ b/src/main.c
@@ -398,7 +398,7 @@ int main(int argc, char **argv)
 	elf_version(EV_CURRENT);
 
 	/* Open the ELF file */
-	ep = elf_patch_open(cmd_opts.elf_path, cmd_opts.dry_run);
+	ep = elf_patch_open(cmd_opts.elf_path, cmd_opts.dry_run, cmd_opts.little_endian);
 	if (!ep) {
 		ret = -2;
 		goto free_cmds;