From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/7754 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: ppc soft-float regression Date: Mon, 25 May 2015 09:44:44 +0200 Message-ID: <1432539884.7942.1.camel@inria.fr> References: <20150517181556.GA23050@euler> <20150517195622.GA4761@euler> <20150518183929.GA6370@euler> <20150518201043.GX17573@brightrain.aerifal.cx> <20150518201422.GY17573@brightrain.aerifal.cx> <20150518220731.GA31132@euler> <20150522062346.GK17573@brightrain.aerifal.cx> <20150524030809.GA19134@brightrain.aerifal.cx> <20150525003648.GO17573@brightrain.aerifal.cx> <1432535489.2715.1.camel@inria.fr> <20150525065756.GR17573@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature"; boundary="=-5EjcRcLEM/5VT07N8J2N" X-Trace: ger.gmane.org 1432539910 7196 80.91.229.3 (25 May 2015 07:45:10 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 25 May 2015 07:45:10 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-7766-gllmg-musl=m.gmane.org@lists.openwall.com Mon May 25 09:44:58 2015 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1Ywn4E-0000QE-EB for gllmg-musl@m.gmane.org; Mon, 25 May 2015 09:44:58 +0200 Original-Received: (qmail 32552 invoked by uid 550); 25 May 2015 07:44:57 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 32528 invoked from network); 25 May 2015 07:44:56 -0000 X-IronPort-AV: E=Sophos;i="5.13,490,1427752800"; d="scan'";a="153707598" In-Reply-To: <20150525065756.GR17573@brightrain.aerifal.cx> Face: 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Cj15cu23XZI4/826Px/xqB37z547sPH374oOvnKTsOb+1zuCoImZARM8ZiLlKkXcWOcoFRJ+Do/CwFO4e2DEe+9Ti+ZXA8ny/cU+X4bMKRZxZIVEiqXI6/+NKuUzPHb7z40ovnX3zx+Ctuq38HyuqWG7Tu+A0AAAAASUVORK5CYII= X-Mailer: Evolution 3.12.9-1+b1 Xref: news.gmane.org gmane.linux.lib.musl.general:7754 Archived-At: --=-5EjcRcLEM/5VT07N8J2N Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Am Montag, den 25.05.2015, 02:57 -0400 schrieb Rich Felker: > On Mon, May 25, 2015 at 08:31:29AM +0200, Jens Gustedt wrote: > > 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. Whil= e > > > 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. > > >=20 > > > Here're the basic changes I'd like to make to dlstart.c to implement > > > this: > > >=20 > > > 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. > > >=20 > > > 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. > > >=20 > > > 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. > > >=20 > > > 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. > > >=20 > > > 5. Have the do_relocs in stage 3 (for ldso/libc only) pull addends > > > from this array instead of of from inline. > > >=20 > > > 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. > > >=20 > > > Comments? > >=20 > > I like it. > >=20 > > 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 > >=20 > > - a sanity check > > - an alternative strategy if the sanity check fails > >=20 > > Would there be a possibility to have both here, too? >=20 > I'm not sure what you're referring to in pthread_create. It has no > VLA. Maybe you mean the choice of whether to put TLS in a > caller-provided thread stack, that's what I meant > but I don't think that's an analogous > situaton. In that one, the provided stack size is known and TLS is > arbitrarily large (under the control of the app and loaded libs). Here > (dlstart), the stack size is not known and the size of the addend > table is not specifically known, but it's fixed at ld-time for libc.so > and the same for every run, and it's orders of magnitude smaller than > any usable stack (e.g. 14 entries on i386). > There are two potential failure modes here: >=20 > 1. We run out of stack because RLIMIT_STACK is insanely small. In that > case we'll quickly crash somewhere else. If there's a risk of > getting off the stack into other memory, that risk would already > exist in other places. >=20 > 2. We run out of stack because the REL table is HUGE. This is a static > condition for the libc.so ELF file and would not change from run to > run. If something went wrong in the build process to cause this, it > needs to be fixed. >=20 > So I'm skeptical of the need for a fallback. >=20 > If we do want/need a fallback, it would need to be of the following > form: >=20 > 1. Count addends that need to be saved. >=20 > 2. If cnt produce VLA of size 1 and pass a null pointer instead of a pointer > to the VLA. Ok, so we have a simple sanity check, great. > 3. In stage 2, if the addend-buffer pointer is null, allocate storage > for it with __syscall(SYS_mmap, ...) (we can't call mmap yet). > Abort on failure. >=20 > 4. In stage 3, if the addend-buffer was allocated donate it to malloc > after we're done using it. >=20 > I would probably prefer just having it crash/abort in step 3, me too > since > this condition obviously indicates a broken build. It's better to > catch such an issue and fix it than to have a non-robust libc.so that > breaks fail-safety for no-third-party-libs binaries by depending on > allocation. Observe that I said "alternative strategy" not "fallback". "Crash early" to be that strategy sounds completely ok to me. Thanks Jens --=20 :: 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 :: --=-5EjcRcLEM/5VT07N8J2N Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iEYEABECAAYFAlVi0uwACgkQD9PoadrVN+LMjwCeLTL/bQ7IRm7IbxRGeYf/mX4J RVcAoIZRHVfDnu7KmPKtcck9+/AGr0/9 =pDdY -----END PGP SIGNATURE----- --=-5EjcRcLEM/5VT07N8J2N--