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

[Carlos O'Donell]

On 10/23/2014 12:40 PM, Will Newton wrote:
> 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?

Just a reproducer. Please try to make it minimal. For example my reproducer
here was pretty small, install malloc hook, dlopen something, have malloc
hook dlopen something else, boom.

Cheers,
Carlos.