[PATCH v6 07/13] ARC: Linux Syscall Interface

[Adhemerval Zanella]

On 03/06/2020 16:46, Vineet Gupta wrote:
> On 5/29/20 9:49 AM, Adhemerval Zanella via Libc-alpha wrote:
>>> +	; ----- child starts here ---------
>>> +
>>> +	; Setup TP register (only recent kernels v4.19+ do that)
>>> +	and.f	0, r12, CLONE_SETTLS
>>> +	mov.nz	r25, r9
>> Do you still need to set it since the minimum supported kernel
>> for ARC is 5.1 ?
> 
> Right.
> 
>> It should be safe for internal glibc usage, since for both pthread
>> and posix_spawn it blocks all signals including SIGCANCEL and SIGXID.
>> However this is still small race window if this is called directly 
>> with pthread cancellation or g*uid in multithread.
> 
> I'm not sure what you mean above. Do you mean not doing this in glibc and even if
> kernel support didn't exist should be safe internally ?

At least for internal clone usage with CLONE_VM within glibc we explicit
disable all signals (posix_spawn and pthread_create).

> 
> fwiw as mentioned above kernel sets up TP for clone (SETTLS). I detested doing
> that for a long time, give ABI implications but ended up doing it anyways due to
> an actual race hit when running uClibc tst-kill6 [1]

We explicit disable all signals during the create_thread call in pthread_create
(b3cae39dcbfa2432b3f3aa28854d8ac57f0de1b8), so it should not happen on glibc
anymore.  However it is still an issue if application calls clone itself.

> 
> [1] http://lists.infradead.org/pipermail/linux-snps-arc/2018-October/004480.html
>