Re: RFC: Audit external function called indirectly via GOT

[Florian Weimer <fweimer@redhat.com>]

On 03/20/2018 05:52 PM, H.J. Lu wrote:
> On Mon, Mar 19, 2018 at 1:21 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 03/17/2018 02:31 PM, H.J. Lu wrote:
>>>
>>> Auditing of external function calls and their return values relies on
>>> lazy binding with PLT.  When external functions are called indirectly
>>> via GOT without using PLT, auditing stops working.
>>>
>>> Here is a proposal to support auditing of external function called
>>> indirectly via GOT:
>>>
>>> 1. Add optional dynamic tags:
>>>
>>>    #define DT_GNU_PLT     0x6ffffef4  /* Address of PLT section  */
>>>    #define DT_GNU_PLTSZ   0x6ffffdf1  /* Size of PLT section  */
>>>    #define DT_GNU_PLTENT  0x6ffffdf2  /* Size of one PLT entry  */
>>>    #define DT_GNU_PLT0SZ  0x6ffffdf3  /* Size of the first PLT entry  */
>>>    #define DT_GNU_PLTGOTSZ 0x6ffffdf4 /* Size of PLTGOT section  */
>>>
>>> and update DT_FLAGS_1 with:
>>>
>>>    #define DF_1_JMPRELIGN 0x10000000  /* DT_JMPREL can be ignored  */
>>> 2. Linker creates PLT entries for auditing external function calls via
>>> GOT and sets DT_GNU_PLT, DT_GNU_PLTSZ, DT_GNU_PLTENT, DT_GNU_PLT0SZ and
>>> DT_GNU_PLTGOTSZ.  If PLT isn't required for lazy binding, set the
>>> DF_1_JMPRELIGN bit in DT_FLAGS_1.
>>
>>
>> Could we ship a template for the PLT entries in ld.so instead?  And if
>> needed, map it from the file together with an address array, like this?
> 
> This won't work since linker needs to know exactly PLT layout to generate
> JUMP_SLOT relocations for LD_AUDIT.

I don't see why it would need JUMP_SLOT relocations if it simply 
auto-generates PLT stub equivalents and installs them in GLOB_DAT 
relocations.

Anyway, going back to the larger question what we need here.

I used  this as a test case for audit support with BIND_NOW:

latrace /bin/true --help

Most of Fedora is compiled with BIND_NOW.  Fedora 26 does not print 
latrace messages (the problem I mentioned earlier), Fedora 27 works 
(yay), Fedora 28 crashes (meh).

So depending on which side Fedora 28+ falls, I think your approach might 
be viable.  I expect that a future binutils version would do this by 
default, and beyond the additional dynamic section tags, new PLT stubs 
would only be created for no-plt functions because current binutils is 
supposed to generate PLT entries again (after they went missing for -z 
now binaries for some time).

Thanks,
Florian