From: Mark Wielaard <mark@klomp.org>
Date: Fri, 18 Oct 2024 14:52:49 +0000 (+0200)
Subject: libelf: Treat elf_memory image as writable
X-Git-Tag: elfutils-0.192~3
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=81981fdfd8f776508579da60d29e068359c61e6a;p=thirdparty%2Felfutils.git

libelf: Treat elf_memory image as writable

There are use cases where the Elf image created by elf_memory is
manipulated, through the libelf interfaces, in place. This doesn't
work anymore since we changed elf_memory to assume the memory is
read-only in elfutils 0.191. commit cc44ac674 ('libelf: Treat
elf_memory as if using ELF_C_READ_MMAP').

The reason for that change was that if elf_memory was given a
read-only memory image then decompressing a section with elf_compress
would crash. Since it directly writes the updated Shdr size. If you do
want to use elf_compress on an Elf created by elf_memory you have make
sure the memory is writable. You can do this for example by using mmap
PROTE_WRITE and MAP_PRIVATE.

	* libelf/elf_memory.c (elf_memory): Call
	__libelf_read_mmaped_file with ELF_C_READ_MMAP_PRIVATE.
	* tests/elfgetzdata.c (main): Use mmap PROT_WRITE and MAP_PRIVATE.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---

diff --git a/libelf/elf_memory.c b/libelf/elf_memory.c
index 13d77cb7..1df49d73 100644
--- a/libelf/elf_memory.c
+++ b/libelf/elf_memory.c
@@ -46,5 +46,6 @@ elf_memory (char *image, size_t size)
       return NULL;
     }
 
-  return __libelf_read_mmaped_file (-1, image, 0, size, ELF_C_READ_MMAP, NULL);
+  return __libelf_read_mmaped_file (-1, image, 0, size,
+				    ELF_C_READ_MMAP_PRIVATE, NULL);
 }
diff --git a/tests/elfgetzdata.c b/tests/elfgetzdata.c
index 0af6c223..a50275fe 100644
--- a/tests/elfgetzdata.c
+++ b/tests/elfgetzdata.c
@@ -69,7 +69,8 @@ main (int argc, char *argv[])
       else
         {
 	  assert (do_mem);
-	  // We mmap the memory ourselves, explicitly PROT_READ only
+	  // We mmap the memory ourselves, explicitly PROT_READ | PROT_WRITE
+	  // elf_memory needs writable memory when using elf_compress.
 	  struct stat st;
 	  if (fstat (fd, &st) != 0)
 	    {
@@ -79,7 +80,8 @@ main (int argc, char *argv[])
 	      continue;
 	    }
 	  map_size = st.st_size;
-	  map_address = mmap (NULL, map_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	  map_address = mmap (NULL, map_size, PROT_READ | PROT_WRITE,
+			      MAP_PRIVATE, fd, 0);
 	  if (map_address == MAP_FAILED)
 	    {
 	      printf ("%s cannot mmap %s\n", argv[cnt], strerror (errno));