Should glibc be fully reentrant? What do we allow interposed symbols to do?

[Will Newton]

On 23 October 2014 15:40, Carlos O'Donell <carlos@redhat.com> wrote:
> On 10/23/2014 04:23 AM, Will Newton wrote:
>> On 23 October 2014 03:29, Carlos O'Donell <carlos@redhat.com> wrote:
>>> On 10/21/2014 06:47 PM, Roland McGrath wrote:
>>>> But the slippery slope concerns me most of all.
>>>
>>> Any function the user interposes acts as a synchronous interrupt on the
>>> runtime.
>>>
>>> It is my opinion that users expect to be able to call any routine in the
>>> runtime without caution unless we tell them otherwise.
>>>
>>> Given that dlopen locks are recursive, as are stdio locks, I propose we
>>> fully support this notion that users already believe exists.
>>>
>>> The alternative is that we don't support it and treat interposed functions
>>> as if they were in a signal handler context, only being allowed to call
>>> async-signal-safe functions, and we might as well remove the recursive
>>> support from the locks such that users get useful backtraces from deadlocks.
>>> It is my opinion that such a direction would not help our users and would
>>> not help the project.
>>>
>>> The similar situations we need to clarify are LD_AUDIT modules, and
>>> IFUNC resolvers, but let us proceed orderly one topic at a time.
>>>
>>> In summary
>>> ==========
>>>
>>> Allow interposed functions to call back into the runtime, and fix any
>>> places where this breaks.
>>
>> Do you have a plan to fix dlsym similarly? ISTR that pretty reliably
>> trips on the same issue when used in a malloc hook.
>
> If you can write a test case I will look at it.

I'll see what I can do. Are you looking for a glibc testcase or just a
reproducer?

-- 
Will Newton
Toolchain Working Group, Linaro