From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pj1-x102e.google.com (mail-pj1-x102e.google.com [IPv6:2607:f8b0:4864:20::102e]) by sourceware.org (Postfix) with ESMTPS id A51F13853540 for ; Wed, 8 Jun 2022 01:51:55 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org A51F13853540 Received: by mail-pj1-x102e.google.com with SMTP id w13-20020a17090a780d00b001e8961b355dso5295924pjk.5 for ; Tue, 07 Jun 2022 18:51:55 -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=C0VRPz5mxRdM08gZv188a4mnfOL0ULu80Zv7W/omjj0=; b=TR6i887qW9DQ0X6yecfJg25rrdKsj11LOYIkj6MRzmg5YCk7wPlolTD8bv5n+j7XDA stdjzDr1/SWgAT/FFsEsMdwuP9qbUAqBUxRvqja5mM/zkg2TBFcRpUPiHXLHX9tZB1Xv LyxqhU+zSB8EY7ML5FfGbDndZRvMhruQUA5P68Zyi4ufDBrlNTb+aP5P7eBJIhHkdjCJ UsurqDAIitWClSamp3zoj+oifiA64+xmWh63AV1nMDo8JRmQcnR1WFSxVPphPMuL5oOy MyOwBoWARKPjug0UsGul/DhCkgPIcWDQTOdFT74CpcIekzi4e14JUvCOjAphzKUtXhPB nFMQ== X-Gm-Message-State: AOAM532ojLebhUuQFyGADnP7aF5znmCyvcbvH3vMzKy/9wDkdZ75vDyV pXxB0vOFleaXctyGi85X6+pCRRJ8G3sT07VZ6cy3+yYum8M= X-Google-Smtp-Source: ABdhPJxanWNFCFKJM++TMqeA9dUwpzEp5ICRac74J00ylWa0jfWz3ryRVBsF7Wf7pT9REVlqmdc20NbZcUyHQq1vXpI= X-Received: by 2002:a17:90b:2349:b0:1e3:34f9:87e8 with SMTP id ms9-20020a17090b234900b001e334f987e8mr45342964pjb.217.1654653114687; Tue, 07 Jun 2022 18:51:54 -0700 (PDT) MIME-Version: 1.0 References: <20220601175633.2407189-1-maskray@google.com> <20220607182135.3ahsg3mu6nxh3pee@google.com> <20220607200005.7bz5bln6ogadenlh@google.com> <20220607235720.dd6xsxyb32hwwnrb@google.com> In-Reply-To: <20220607235720.dd6xsxyb32hwwnrb@google.com> From: "H.J. Lu" Date: Tue, 7 Jun 2022 18:51:18 -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=-3025.1 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: Wed, 08 Jun 2022 01:51:58 -0000 On Tue, Jun 7, 2022 at 4:57 PM Fangrui Song wrote: > > > 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. Does lld always set non-zero symbol values for undefined function symbols? > >> >> >> > 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. -- H.J.