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Re: RFC: Audit external function called indirectly via GOT
On Wed, Mar 28, 2018 at 11:37 AM, Florian Weimer <fweimer@redhat.com> wrote:
> 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.
My understanding is that LD_AUDIT is based on JUMP_SLOT 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).
>
-fno-plt is a compiler option, not a linker option. Linker generates PLT for
PLT32 relocations to external functions.
--
H.J.