Date: Mon, 25 May 2015 08:31:29 +0200 From: Jens Gustedt <jens.gustedt@...ia.fr> To: musl@...ts.openwall.com Subject: Re: ppc soft-float regression Am Sonntag, den 24.05.2015, 20:36 -0400 schrieb Rich Felker: > There's a simple alternative I just came up with though: have > dlstart.c compute the number of REL entries that need their addends > saved and allocate a VLA on its stack for stages 2 and 3 to use. While > the number of addends could be significant, it's many orders of > magnitude smaller than the smallest practical stack sizes we could > actually run with, so it's perfectly safe to put it on the stack. > > Here're the basic changes I'd like to make to dlstart.c to implement > this: > > 1. Remove processing of DT_JMPREL REL/RELA table. All entries in this > table are necessarily JMP_SLOT type, and thus symbolic, so there's > nothing stage 1 can do with them anyway. Also, being JMP_SLOT type, > they have implicit addends of zero if they're REL-type, so there's > no need to save addends. > > 2. Remove the loop in dlstart.c that works like a fake function call 3 > times to process DT_JMPREL, DT_REL, and DT_RELA. Instead we just > need 2 iterations, and now the stride is constant in each, so they > should simplify down a lot more inline. > > 3. During the loop that processes DT_REL, count the number of > non-relative relocations (ones we skip at this stage), then make a > VLA this size and pass its address to __dls2 as a second argument. > > 4. Have the do_relocs in stage 2 save addends in this provided array > before overwriting them, and save its address for use by stage 3. > > 5. Have the do_relocs in stage 3 (for ldso/libc only) pull addends > from this array instead of of from inline. > > Steps 1 and 2 are purely code removal/simplification and should be > done regardless of whether we move forward on the above program, I > think. Steps 3-5 add some complexity but hardly any code, just a few > lines here and there. > > Comments? I like it. The thing that is a bit critical here, is the VLA. Not because it is a VLA as such, but because it is a dynamic allocation on the stack. We already have a similar strategy in pthread_create for TLS. The difference is that there we have - a sanity check - an alternative strategy if the sanity check fails Would there be a possibility to have both here, too? Thanks Jens -- :: INRIA Nancy Grand Est ::: Camus ::::::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: Download attachment "signature.asc" of type "application/pgp-signature" (182 bytes)
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.