From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pj1-x1036.google.com (mail-pj1-x1036.google.com [IPv6:2607:f8b0:4864:20::1036]) by sourceware.org (Postfix) with ESMTPS id E50DE3857376 for ; Tue, 7 Jun 2022 23:57:25 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org E50DE3857376 Received: by mail-pj1-x1036.google.com with SMTP id hv24-20020a17090ae41800b001e33eebdb5dso14877780pjb.0 for ; Tue, 07 Jun 2022 16:57:25 -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=GKlct54wxpa1N+hdoEcy1m/3GHzOraXTAtETdqiu54Y=; b=MFJX/hvL7MHXK7rRs/d0Cs+r3OQ3Kflvcgz7tFBun6RTKKg3dShO6smIz0W68b4Q3z y4KJCrYLPfzAFBkxI//m71r6y6gvT5nx9iQfbuIOAN+uILT6bYSaujy46QoeBHfLnM+B eCTPpVg3EROJSnE8XrJogpSaXK0O8zN6qt2iyOyWPgkoy5c2lgWwN4TJayIdoeVTZtDz ZuNwzGYpPtg3Rnmf/dT4lqjKU+ICSIORuX1YG9C5jjh3pCH/qRnvPVFcckHTi59Nskb6 JRUoYCiJ07ugW0aWzSvGEYrUT8jHtQ0AwoJEAR8R1vUY52wCN8nRlPjr9jY/cqPdNNz2 9MTg== X-Gm-Message-State: AOAM532knkfbJus4ED6hrioFLo7rpkN/SSXyJk/hyIwXtyqsoyHJxkXt Q1I0huREign0LYbNEbFUXrC2DA== X-Google-Smtp-Source: ABdhPJwrwDGvXLk12ABXFqB9I3/tn9pt3Xffmv8LPobk90NJ4Fd/f7RN3ZxRXwKp97HNtlGVMr27fw== X-Received: by 2002:a17:90b:304:b0:1e3:4285:ce1e with SMTP id ay4-20020a17090b030400b001e34285ce1emr35495811pjb.71.1654646244655; Tue, 07 Jun 2022 16:57:24 -0700 (PDT) Received: from google.com ([2620:15c:2ce:200:dfe:1f0f:4f14:c886]) by smtp.gmail.com with ESMTPSA id k20-20020a17090aaa1400b001dc37aef4ffsm12441799pjq.48.2022.06.07.16.57.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Jun 2022 16:57:23 -0700 (PDT) Date: Tue, 7 Jun 2022 16:57:20 -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: <20220607235720.dd6xsxyb32hwwnrb@google.com> References: <20220601175633.2407189-1-maskray@google.com> <20220607182135.3ahsg3mu6nxh3pee@google.com> <20220607200005.7bz5bln6ogadenlh@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 23:57:28 -0000 On 2022-06-07, H.J. Lu wrote: >On Tue, Jun 7, 2022 at 1:00 PM Fangrui Song wrote: >> >> On 2022-06-07, H.J. Lu wrote: >> >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? >> >> This plan sounds good, when we create a separate patch enhancing the >> diagnostics. Created https://sourceware.org/pipermail/libc-alpha/2022-June/139552.html ([PATCH] elf: Refine direct extern access diagnostics to protected symbol). >> x86-32 may need a exception (i.e. no warning) for ELF_RTYPE_CLASS_PLT to handle R_386_PC32. > >Linker sets non-zero symbol values for undefined function symbols in >executable only when their addresses are taken. R_386_PC32 shouldn't >matter. OK, I believe GNU ld distinguishes branch/address-taken usages of R_386_PC32 by poking at the instruction opcode. That works. ld.lld doesn't check the opcode, and just reports "error: cannot preempt symbol:" in an example I crafted. >> >> >> > 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. > > > >-- >H.J.