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RE: readelf reporting of e_shstrndx is slightly wrong
From: |
Mike Murphy |
Subject: |
RE: readelf reporting of e_shstrndx is slightly wrong |
Date: |
Tue, 21 Aug 2018 23:09:04 +0000 |
I think an example would help explain this. Below is part of the output from
readelf -h on an elf object I have which has 210016 sections, but puts the
section header string table at section 1. So e_shstrndx is 1, but e_shnum is
0. My reading of the elf standard is that this is legal, but readelf complains
due to the check header->e_shstrndx >= header->e_shnum:
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 0 (210016)
Section header string table index: 1 <corrupt: out of range>
The code in readelf seems to assume that if there are > 0xff00 sections then
shstrndx will be one of those sections that are > 0xff00.
-----Original Message-----
From: Nick Clifton <address@hidden>
Sent: Tuesday, August 21, 2018 8:32 AM
To: Mike Murphy <address@hidden>; address@hidden
Subject: Re: readelf reporting of e_shstrndx is slightly wrong
Hi Mike,
> If the file has no section name string table, this member holds the value
> |SHN_UNDEF|.
> If the section name string table section index is greater than or
> equal to |SHN_LORESERVE| (|0xff00|), this member has the value
> |SHN_XINDEX| (|0xffff|)
> The current readelf -h seems to assume that if there are more than 0xff00
> sections, then the shstrndx will also be past that. But there is nothing to
> prevent the section name string table from being section 1, in which case
> e_shstrndx should just be 1. But the readelf implementation has:
>
> else if (elf_header.e_shstrndx != SHN_UNDEF &&
> elf_header.e_shstrndx >= elf_header.e_shnum)
>
> printf(_("<corrupt: out of range>"));
I disagree. The readelf code actually looks like this:
if (filedata->section_headers != NULL
&& header->e_shstrndx == (SHN_XINDEX & 0xffff))
printf (" (%u)", filedata->section_headers[0].sh_link);
else if (header->e_shstrndx != SHN_UNDEF
&& header->e_shstrndx >= header->e_shnum)
printf (_(" <corrupt: out of range>"));
There is no check that the file itself has more than 0xff00 sections.
Instead it checks to see if the e_shstrndx field is SHN_XINDEX and if so it
follows the link. Otherwise it checks that the index is either SHN_UNDEF or a
valid section number.
Note - the use if "& 0xffff" in the above code is confusing, and looks surplus
to me, but I do not think that it makes any difference to the behaviour.
Cheers
Nick