Date: Tue, 29 May 2018 08:33:17 +0200 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: TLS issue on aarch64 * Rich Felker <dalias@...c.org> [2018-05-28 18:15:21 -0400]: > On Mon, May 28, 2018 at 10:47:31PM +0200, Szabolcs Nagy wrote: > > another issue with the patch is that if tp is aligned then pthread_t > > may not get aligned: > > > > tp == self + sizeof(pthread_t) - reserved > > This expression does not look correct; it would have tp point > somewhere inside of the pthread structure rather than just past the > end of it. > this is the current tp setup on aarch64, arm and sh4, see TP_ADJ we can just make tp = self+sizeof(pthread_t) but then there will be a small unused hole > Maybe your code is doing this to avoid wasted padding, but if so I > think that's a bad idea. It violates the invariant that the pthread > structure is at a constant offset from tp, which is needed for > efficient pthread_self and for access to fixed slots at the end of it > (like canary or dtv copy). > > > so sizeof(pthread_t) - reserved must be divisible with > > gcd(alignment of tp, alignof(pthread_t)) to be able to make both > > self and tp aligned. > > > > this is not an issue on current targets with current pthread_t, > > but we may want to decouple internal struct pthread alignment > > details and the abi reserved tls size, i.e. tp_adj could be like > > > > tp == alignup(self + sizeof(pthread_t) - reserved, alignof(pthread_t)) > > > > or we add a static assert that reserved and alignof(pthread_t) > > are not conflicting. > > Maybe I'm misunderstanding what "reserved" is, since you're talking > about a static assert...? > it is the abi reserved space after tp (the bfd linker calls it TCB_SIZE)
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