Re: [Elftoolchain-developers] Support for "in-place" updates to ELF objects has been added to libelf

[Eric S. <sch...@gm...>]

Joseph Koshy <jk...@us...> writes:

>>  $ ./extend-a-section hello
>>  fstat() failed: "Bad address".
>>  Size error: expected=118, elf_update()=6536
>
>> I imagine that I broke something while replacing the TC_* macros
>> with C code.  I apologize for asking such a simple question, but
>> would you mind taking a look to see if the error is obvious to you.
>
> 1) The stat() call at line 46 appears to be failing because argv[2]
>    was not supplied.
>
> File "extend-a-section.c":
>
>     21	  struct stat sb;
>     ...
>     25	  const char *srcfile = argv[1];
>     26	  const char *reffile = argv[2];
>     ...
>     46	  if (stat(reffile, &sb) < 0) {
>     47	    printf("fstat() failed: \"%s\".\n", strerror(errno));
>     48	  }
>
> The value of the "sb.st_size" field would be undefined, and this
> then leads to the failure further down.
>
>     ...
>     64	  if (fsz1 != sb.st_size) {
>     65	    printf("Size error: expected=%ld, elf_update()=%ld\n",
> sb.st_size, fsz1);
>     66	  }
>
>
> You could remove these file size checks---they are only relevant for test code.
>
> 2) If your code is changing an ELF executable, it would also need to
>    adjust the ELF program header to match the new layout of the file's
>    content---in the ELF(3) API, the contents of the PHDR table are
>    managed by the application, not libelf.
>
> Regards,
> Koshy

Thank you for the advice.

I am now able to read, modify and write an elf file without
corruption.

Specifically, using the attached script I can change the last byte the
.text section (a padding byte which is of no consequence) of a hello
world file.  I can then write the modified file out to disk resulting in
a new valid elf file which is different from the original but still
executes successfully.

Thanks for the help -- Eric