From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-x630.google.com (mail-pl1-x630.google.com [IPv6:2607:f8b0:4864:20::630]) by sourceware.org (Postfix) with ESMTPS id 1675F3857413 for ; Tue, 7 Jun 2022 19:22:28 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 1675F3857413 Received: by mail-pl1-x630.google.com with SMTP id b5so15618445plx.10 for ; Tue, 07 Jun 2022 12:22:28 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=2KdeOaCX/owSSk9peuWimFp++LYa4P3cwB3s1x2pQGs=; b=VAm7amaW5ATJSSEfR5Z6DJucQ9HBOKNQ6WuMfCSDP7HjJL9FYXPQpqzgfTe4vgDaYA y+7NLwNI2FSjCkKicl3aKocctAF0rpHGXvpxI3gKy1wbykVeOhNPeITOhmi/vFArh6gH rL67tbA75xn2Z10yWN4ch2sWOjifAoIGt1BHEYiB+rTz4LHbT1CJFR76vtANepHEqnEo +LMJs5smTr+Fc25ktgrlygZD+wczrIIpPx/Spg1KtDEd9KtH9P50y0sX1eZq6gu3SI97 /UccUEJ5nAuGJfKSYrOmD1GsEMVfLwU0yLf7kSVgHsXzRpCbnqpm+G+zEJ5usMgnMDo5 aPQA== X-Gm-Message-State: AOAM533LQVNGO/J+nnWw1V7WDsa50CK1AXsCqWiNU7lkq7QJzaAHJF4o bpToAEcSvbofAqXkMr5bwcnxXncow4qv10GJBuJZh8dYUCs= X-Google-Smtp-Source: ABdhPJwdT+RpvCG6MDrg+1A28sNp4qan5k6Q7SwWcaI1kBVF8FFn8AAQm3RTCmfIK4eLk55BPO7vUGzE21Lom80Gtqs= X-Received: by 2002:a17:902:7088:b0:167:78c0:e05e with SMTP id z8-20020a170902708800b0016778c0e05emr12863715plk.149.1654629747022; Tue, 07 Jun 2022 12:22:27 -0700 (PDT) MIME-Version: 1.0 References: <20220601175633.2407189-1-maskray@google.com> <20220607182135.3ahsg3mu6nxh3pee@google.com> In-Reply-To: <20220607182135.3ahsg3mu6nxh3pee@google.com> From: "H.J. Lu" Date: Tue, 7 Jun 2022 12:21:51 -0700 Message-ID: Subject: Re: [PATCH v3] elf: Remove ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA To: Fangrui Song Cc: Szabolcs Nagy , GNU C Library Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-3025.6 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, KAM_INFOUSMEBIZ, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 07 Jun 2022 19:22:30 -0000 On Tue, Jun 7, 2022 at 11:21 AM Fangrui Song wrote: > > On 2022-06-07, H.J. Lu wrote: > >On Tue, Jun 7, 2022 at 6:25 AM Szabolcs Nagy via Libc-alpha > > wrote: > >> > >> The 06/01/2022 10:56, Fangrui Song wrote: > >> > If an executable has copy relocations for extern protected data, that > >> > can only work if the library containing the definition is built with > >> > assumptions (a) the compiler emits GOT-generating relocations (b) the > >> > linker produces R_*_GLOB_DAT instead of R_*_RELATIVE. Otherwise the > >> > library uses its own definition directly and the executable accesses a > >> > stale copy. Note: the GOT relocations defeat the purpose of protected > >> > visibility as an optimization, but allow rtld to make the executable and > >> > library use the same copy when copy relocations are present, but it > >> > turns out this never worked perfectly. > >> > > >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA has strange semantics when both > >> > a.so and b.so define protected var and the executable copy relocates > >> > var: b.so accesses its own copy even with GLOB_DAT. The behavior change > >> > is from commit 62da1e3b00b51383ffa7efc89d8addda0502e107 (x86) and then > >> > copied to nios2 (ae5eae7cfc9c4a8297ff82ec6b794faca1976ecc) and arc > >> > (0e7d930c4c11de896fe807f67fa1eb756c9c1e05). > >> > > >> > Without ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA, b.so accesses the copy > >> > relocated data like a.so. > >> > > >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA has another effect in the absence > >> > of copy relocations: when a protected data symbol is defined in multiple > >> > objects, the code tries to bind the relocation locally. Without > >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA, STV_PROTECTED is handled in the > >> > same way as STV_DEFAULT: if ld produces GLOB_DAT (some ports of GNU ld), > >> > the relocation will bind to the first definition; otherwise (e.g. > >> > ld.lld) ld does the binding locally and ld.so doesn't help. > >> > > >> > >> i think we should not change the interposition semantics. > >> we should go back to the old behaviour where only copy > >> relocs were broken (and there was an expensive workaround > >> to deal with protected symbol interposition). > >> > >> i think you want to revert the elf/dl-lookup.c changes of > >> > >> commit 62da1e3b00b51383ffa7efc89d8addda0502e107 > >> Author: H.J. Lu > >> CommitDate: 2015-03-31 05:16:57 -0700 > >> > >> Add ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA to x86 > >> > > > >I am OK to remove support of copy relocation against protected > >symbols since it doesn't work properly. > > Thanks. > > >My only question is if > >ld.so should issue a warning or an error when seeing a copy > >relocation against a protected symbol. Copy relocation against > >protected symbol defeats the purpose of protected symbol. > > The check already exists (_dl_check_protected_symbol) but currently > relies on GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS (only implemented > for x86, and adoption is low on x86). > > For ELF_RTYPE_CLASS_COPY, I think the GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS > check can be removed. Will removal of GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS check cause many run-time errors? > ( > Since GCC 5, x86-64 -fpie has HAVE_LD_PIE_COPYRELOC. > When neither -m[no]direct-extern-access is specified, HAVE_LD_PIE_COPYRELOC takes effect. > The executable does not have GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS > but the incompatibility exists. > It just kinda works because GCC and GNU ld cooperate to produce a GLOB_DAT in the DSO. > ) > > For ELF_RTYPE_CLASS_PLT, the pointer equality does not matter much in > practice: > > * protected visibility adoption is very low due to various problems. > * Taking a function address in the executable and expecting it to match the address in a DSO is rare. > * Many users use ICF and by and large don't care about function addresses to some extent. > > I think having the warning under GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS is fine. > ( > * x86-32 -fno-pic uses R_386_PC32 as a jump instruction, which is > indistinguishable from an address-taken operation > https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected#branch-instructions-on-x86 > ) An error with GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS and a warning without? > > >> > It's extremely unlikely anyone relies on the > >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA behavior, so let's remove it: this > >> > removes a check in the symbol lookup code. > >> > > >> > -- > >> > Changes from v1: > >> > * Reword commit message as suggested by Szabolcs Nagy > >> > > >> > Changes from v2: > >> > * Explain interposition behavior > >> > --- > >> > elf/dl-lookup.c | 90 ------------------------------------- > >> > sysdeps/arc/dl-sysdep.h | 21 --------- > >> > sysdeps/generic/ldsodefs.h | 12 +---- > >> > sysdeps/i386/dl-machine.h | 3 +- > >> > sysdeps/nios2/dl-sysdep.h | 21 --------- > >> > sysdeps/x86/dl-lookupcfg.h | 4 -- > >> > sysdeps/x86_64/dl-machine.h | 8 +--- > >> > 7 files changed, 4 insertions(+), 155 deletions(-) > >> > delete mode 100644 sysdeps/arc/dl-sysdep.h > >> > delete mode 100644 sysdeps/nios2/dl-sysdep.h > >> > > >> > diff --git a/elf/dl-lookup.c b/elf/dl-lookup.c > >> > index a42f6d5390..41d108e0b8 100644 > >> > --- a/elf/dl-lookup.c > >> > +++ b/elf/dl-lookup.c > >> ... > >> > @@ -854,43 +801,6 @@ _dl_lookup_symbol_x (const char *undef_name, struct link_map *undef_map, > >> > return 0; > >> > } > >> > > >> > - int protected = (*ref > >> > - && ELFW(ST_VISIBILITY) ((*ref)->st_other) == STV_PROTECTED); > >> > - if (__glibc_unlikely (protected != 0)) > >> > - { > >> > - /* It is very tricky. We need to figure out what value to > >> > - return for the protected symbol. */ > >> > - if (type_class == ELF_RTYPE_CLASS_PLT) > >> > - { > >> > - if (current_value.s != NULL && current_value.m != undef_map) > >> > - { > >> > - current_value.s = *ref; > >> > - current_value.m = undef_map; > >> > - } > >> > - } > >> > - else > >> > - { > >> > - struct sym_val protected_value = { NULL, NULL }; > >> > - > >> > - for (scope = symbol_scope; *scope != NULL; i = 0, ++scope) > >> > - if (do_lookup_x (undef_name, new_hash, &old_hash, *ref, > >> > - &protected_value, *scope, i, version, flags, > >> > - skip_map, > >> > - (ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA > >> > - && ELFW(ST_TYPE) ((*ref)->st_info) == STT_OBJECT > >> > - && type_class == ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA) > >> > - ? ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA > >> > - : ELF_RTYPE_CLASS_PLT, NULL) != 0) > >> > - break; > >> > - > >> > - if (protected_value.s != NULL && protected_value.m != undef_map) > >> > - { > >> > - current_value.s = *ref; > >> > - current_value.m = undef_map; > >> > - } > >> > - } > >> > - } > >> > - > >> > >> i think we should keep this part without the > >> ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA bit. > > > > > > > >-- > >H.J. -- H.J.