Re: [Elftoolchain-developers] Error adding section to existing ELF binary

[Frank T. <fra...@ts...>]

Dear all,

I tried version 0.6.1 of the elftoolchain (directly compiled from the
sourceforge repo) and the bug seems to be fixed.

But now I am facing another issue.

If the custom section is of type SHT_LOUSER, the current implementation
of elf_update() fails.

I tracked down the issue to the function _libelf_xlate_shtype() which
does not have a proper case statement for the type SHT_LOUSER. As this
function call fails, the elf_update() function aborts with an error
indicating an invalid section type.

I quick-fixed the issue in the function _libelf_xlate_shtype() by
removing the default-branch of the case statement and adding an addition
if-clause to return the correct ELF type (ELF_T_BYTE).

--------------------------------

switch (sht) {
...
case SHT_SUNW_verneed:	/* == SHT_GNU_verneed */
	return (ELF_T_VNEED);
case SHT_SUNW_versym:	/* == SHT_GNU_versym */
	return (ELF_T_HALF);
}

if (sht >= SHT_LOUSER && sht <= SHT_HIUSER) {
	return (ELF_T_BYTE);
}

return -1;
	
--------------------------------

I guess that an additional statement may be needed for SHT_LOPROC,
SHT_HIOS or similar type which are user-defined.

Regards,

Frank Thater



Am 24.07.2014 12:25, schrieb Frank Thater:
> Dear all,
> 
> I am trying to add a new section with an application specific content to
> an existing ELF file. Unfortunately the resulting ELF file is erroneous
> as the size/offset of the new section and the section type are not
> correctly set (see code snippet below).
> 
> "Readelf" indicates the following information (section 37 is the newly
> added section):
> 
> ...
> [36] .strtab           STRTAB          00000000 013ccc 000356 00      0
>   0  1
> [37]                   NULL            00000000 000000 8048154 8048168
>     0 65539  0
> 
> The type of the section (NULL) and the size/offset (should be 8 bytes
> starting at offset 0x14022) are wrong. I have manually verified the
> correct values within the data structures using a debugger. So I would
> guess that something gets broken during the final call to elf_update().
> 
> OS is Kubuntu 12.04 with package 'libelf1" installed (Version indicated
> is 0.152-1ubuntu).
> 
> I've read some old postings regarding the libelf support for ELF_C_RDWR
> which recommended to copy the sections to a new file due to a broken
> implementation. Are these bugs still present?
> 
> Or am I just missing something? Any hints?
> 
> 
> Frank
> 
> 
> --------------------------------------------------------------------------------------
> 
> 	int fd;
> 	Elf *elf_ref = NULL;
> 	Elf32_Ehdr *ehdr;
> 	Elf_Scn *elf_scn, *sig_scn;
> 	Elf_Data *elf_data;
> 	Elf32_Shdr *signature_scn_header;
> 	unsigned char sample_data[] = {0xca, 0xfe, 0xba, 0xbe, 0xca, 0xfe,
> 0xba, 0xbe};
> 
> 	if (argc != 2) {
> 		return -1;
> 	}
> 
> 	fd = open(argv[1], O_RDWR);
> 
> 	/* Protect from using a lower ELF version and initialize ELF library */
> 	if (elf_version(EV_CURRENT) == EV_NONE) {
> 		printf("ELF library init failed: %s\n", elf_errmsg(-1));
> 		close(fd);
> 		return -1;
> 	}
> 
> 	elf_ref = elf_begin(fd, ELF_C_RDWR, NULL);
> 
> 	if (elf_kind(elf_ref) != ELF_K_ELF) {
> 		printf("Program is not an ELF binary\n");
> 		close(fd);
> 		return -2;
> 	}
> 
> 	sig_scn = elf_newscn(elf_ref);
> 
> 	elf_data = elf_newdata(sig_scn);
> 	elf_data->d_align = 1;
> 	elf_data->d_off = 0LL ;
> 	elf_data->d_buf = sample_data ;
> 	elf_data->d_type = ELF_T_BYTE ;
> 	elf_data->d_size = sizeof(sample_data);
> 	elf_data->d_version = EV_CURRENT ;
> 
> 	signature_scn_header = elf32_getshdr(sig_scn);
> 	signature_scn_header->sh_name = 0;
> 	signature_scn_header->sh_type = SHT_LOUSER + 10;
> 
> 	if (elf_update(elf_ref , ELF_C_NULL ) < 0) {
> 		printf("ELF update failed: %s", elf_errmsg (-1));
> 		return -3;
> 	}
> 
> 	(void) elf_flagshdr(sig_scn, ELF_C_SET, ELF_F_DIRTY);
> 	(void) elf_flagscn(sig_scn, ELF_C_SET, ELF_F_DIRTY);
> 	(void) elf_flagdata(elf_data, ELF_C_SET, ELF_F_DIRTY);
> 	(void) elf_flagehdr(elf_ref, ELF_C_SET, ELF_F_DIRTY);
> 	(void) elf_flagelf(elf_ref, ELF_C_SET, ELF_F_DIRTY);
> 
> 	if (elf_update(elf_ref , ELF_C_WRITE ) < 0) {
> 		printf("ELF update failed: %s", elf_errmsg (-1));
> 		return -4;
> 	}
> 
> 	elf_end(elf_ref);
> 
> 	close(fd);
>