From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-x62c.google.com (mail-pl1-x62c.google.com [IPv6:2607:f8b0:4864:20::62c]) by sourceware.org (Postfix) with ESMTPS id 61F1C394BE17 for ; Tue, 7 Jun 2022 18:21:42 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 61F1C394BE17 Received: by mail-pl1-x62c.google.com with SMTP id o6so10421102plg.2 for ; Tue, 07 Jun 2022 11:21:42 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=qw041YAweHo80OFyF+OgMy3rqRh0nOWLXZhY+fVaE/o=; b=0+zrWuX7xK+Rg8P3CIic8V0PVES+PTdhpxwPwAoc9t/nNhG+Wb20WteVN7a/thgYi1 q+ATaB3teyMgDpjoVGCdZzkH3tpB2lLyjQUEgCl1qT9Sj92pKZWPfSBIS6pypuNY2PgT sKo2dgKu3wPCUJt8OpCoSK4uRmcbIdNj6FOehvUgyGWj+iQyxnWHB1xxe36yG/bv3GMA JES1jPpOVy9y65oF3lrVyfvIa/OxpBjtsh5qs04hWnUlzgJMeg0UHqCZ0v/V/jFivYVk GJcCtXnsMshrtInctl91VOGnGGNZPwcNO4AJqteXzp/I4OPpWKCzjPteZRxmznfXAgsA bCuQ== X-Gm-Message-State: AOAM533LZi2+9KaJzuxzhxLrgwh6VCrSEqvN5UGW+v3t/8SIwBlKRMZ6 mv1qraSWL+6xcqf97tUKBsDVje2GPfZp4A== X-Google-Smtp-Source: ABdhPJzEyrKDCc4X+TFfFAIcOgvbDxR3+bxfrtoQxq6TBITqnIPjvOfZx6qei3cA+pz394qlD5p9ZQ== X-Received: by 2002:a17:902:b418:b0:166:3165:eaff with SMTP id x24-20020a170902b41800b001663165eaffmr30495098plr.96.1654626101138; Tue, 07 Jun 2022 11:21:41 -0700 (PDT) Received: from google.com ([2620:15c:2ce:200:ccc7:2cf6:b776:1e0d]) by smtp.gmail.com with ESMTPSA id m20-20020a056a00165400b0051ba303f1c0sm13096717pfc.127.2022.06.07.11.21.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Jun 2022 11:21:40 -0700 (PDT) Date: Tue, 7 Jun 2022 11:21:35 -0700 From: Fangrui Song To: "H.J. Lu" Cc: Szabolcs Nagy , GNU C Library Subject: Re: [PATCH v3] elf: Remove ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA Message-ID: <20220607182135.3ahsg3mu6nxh3pee@google.com> References: <20220601175633.2407189-1-maskray@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-26.9 required=5.0 tests=BAYES_00, DKIMWL_WL_MED, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, ENV_AND_HDR_SPF_MATCH, GIT_PATCH_0, KAM_INFOUSMEBIZ, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE, USER_IN_DEF_DKIM_WL, USER_IN_DEF_SPF_WL 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 18:21:44 -0000 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. ( 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 ) >> > 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.