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Message-Id: <4B4992FA-A648-46FB-9DD7-48D045867EA4@oflebbe.de>
Date: Mon, 10 Aug 2020 17:45:06 +0200
From: Olaf Flebbe <of@...ebbe.de>
To: Szabolcs Nagy <nsz@...t70.net>
Cc: musl@...ts.openwall.com
Subject: Re: Revisiting sigaltstack and implementation-internal signals
> Am 10.08.2020 um 17:41 schrieb Szabolcs Nagy <nsz@...t70.net>:
>
> * Olaf Flebbe <of@...ebbe.de> [2020-08-10 10:15:13 +0200]:
>> I have some problems to follow the discussion here.
>>
>> It is not about musl to create an alternate stack, it is to *honor* the alternate stack, if the application installed one, for a reason.
>>
>> I am proposing smthg like
>>
>> --- /oss/musl-1.2.1/src/thread/synccall.c
>> +++ /work/musl/src/thread/synccall.c
>> @@ -45,7 +45,7 @@
>> {
>> sigset_t oldmask;
>> int cs, i, r;
>> - struct sigaction sa = { .sa_flags = SA_RESTART, .sa_handler = handler };
>> + struct sigaction sa = { .sa_flags = SA_RESTART|SA_ONSTACK, .sa_handler = handler };
>> pthread_t self = __pthread_self(), td;
>> int count = 0;
>>
>> This will fix the problem with dynamic stacks, like go implements it.
>> If the application does not install one, kernel will ignore SA_ONSTACK. (This is even specified by POSIX, since there is no error condition mentioned in man page specifically for this).
>>
>> Tested with go and a glibc threaded setuid test tst-setuid3.c .
>
> this will fail if an application calls sigaltstack,
> then blocks all user signals that are SA_ONSTACK and
> then deallocates the stack passed to sigaltstack.
>
> it is important to discuss what an application may
> or may not do, because the proposed change observably
> modifies the behaviour.
Deallocating an assigned sigaltstack without resetting sigaltstack is undefined behaviour.
Olaf
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