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Biederman" To: Ard Biesheuvel Cc: linux-kernel@vger.kernel.org, x86@kernel.org, Ard Biesheuvel , Arnd Bergmann , kexec@lists.infradead.org, Nathan Chancellor , Nick Desaulniers , Kees Cook , Bill Wendling , Justin Stitt , Masahiro Yamada References: <20240424155309.1719454-11-ardb+git@google.com> Date: Wed, 24 Apr 2024 15:04:02 -0500 In-Reply-To: <20240424155309.1719454-11-ardb+git@google.com> (Ard Biesheuvel's message of "Wed, 24 Apr 2024 17:53:10 +0200") Message-ID: <87bk5yv87h.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1rzir2-00BaKD-48;;;mid=<87bk5yv87h.fsf@email.froward.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.168.167;;;frm=ebiederm@xmission.com;;;spf=pass X-XM-AID: U2FsdGVkX1/ePpyQ5pyFK6twschP4ln4lEVPApbMIwA= X-SA-Exim-Connect-IP: 68.227.168.167 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-Level: * X-Spam-Virus: No X-Spam-Report: * -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP * 0.8 BAYES_50 BODY: Bayes spam probability is 40 to 60% * [score: 0.4833] * 1.5 XMNoVowels Alpha-numberic number with no vowels * 0.0 T_TM2_M_HEADER_IN_MSG BODY: No description available. * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa03 1397; Body=1 Fuz1=1 Fuz2=1] * 0.4 FVGT_m_MULTI_ODD Contains multiple odd letter combinations X-Spam-DCC: XMission; sa03 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: *;Ard Biesheuvel X-Spam-Relay-Country: X-Spam-Timing: total 546 ms - load_scoreonly_sql: 0.02 (0.0%), signal_user_changed: 4.0 (0.7%), b_tie_ro: 2.8 (0.5%), parse: 0.76 (0.1%), extract_message_metadata: 18 (3.3%), get_uri_detail_list: 2.9 (0.5%), tests_pri_-2000: 33 (6.1%), tests_pri_-1000: 2.1 (0.4%), tests_pri_-950: 1.12 (0.2%), tests_pri_-900: 0.81 (0.1%), tests_pri_-90: 84 (15.4%), check_bayes: 83 (15.2%), b_tokenize: 9 (1.7%), b_tok_get_all: 11 (2.0%), b_comp_prob: 2.4 (0.4%), b_tok_touch_all: 57 (10.5%), b_finish: 0.67 (0.1%), tests_pri_0: 392 (71.8%), check_dkim_signature: 0.57 (0.1%), check_dkim_adsp: 3.0 (0.6%), poll_dns_idle: 0.79 (0.1%), tests_pri_10: 1.67 (0.3%), tests_pri_500: 6 (1.1%), rewrite_mail: 0.00 (0.0%) Subject: Re: [RFC PATCH 0/9] kexec x86 purgatory cleanup X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Ard Biesheuvel writes: > From: Ard Biesheuvel > > The kexec purgatory is built like a kernel module, i.e., a partially > linked ELF object where each section is allocated and placed > individually, and all relocations need to be fixed up, even place > relative ones. > > This makes sense for kernel modules, which share the address space with > the core kernel, and contain unresolved references that need to be wired > up to symbols in other modules or the kernel itself. > > The purgatory, however, is a fully linked binary without any external > references, or any overlap with the kernel's virtual address space. So > it makes much more sense to create a fully linked ELF executable that > can just be loaded and run anywhere in memory. It does have external references that are resolved when it is loaded. Further it is at least my impression that non-PIC code is more efficient. PIC typically requires silly things like Global Offset Tables that non-PIC code does not. At first glance this looks like a code passivization. Now at lot of functionality has been stripped out of purgatory so maybe in it's stripped down this make sense, but I want to challenge the notion that this is the obvious thing to do. > The purgatory build on x86 has already switched over to position > independent codegen, which only leaves a handful of absolute references, > which can either be dropped (patch #3) or converted into a RIP-relative > one (patch #4). That leaves a purgatory executable that can run at any > offset in memory with applying any relocations whatsoever. I missed that conversation. Do you happen to have a pointer? I would think the 32bit code is where the PIC would be most costly as the 32bit x86 instruction set predates PIC being a common compilation target. > Some tweaks are needed to deal with the difference between partially > (ET_REL) and fully (ET_DYN/ET_EXEC) linked ELF objects, but with those > in place, a substantial amount of complicated ELF allocation, placement > and patching/relocation code can simply be dropped. Really? As I recall it only needed to handle a single allocation type, and there were good reasons (at least when I wrote it) to patch symbols. Again maybe the fact that people have removed 90% of the functionality makes this make sense, but that is not obvious at first glance. > The last patch in the series removes this code from the generic kexec > implementation, but this can only be done once other architectures apply > the same changes proposed here for x86 (powerpc, s390 and riscv all > implement the purgatory using the shared logic) > > Link: https://lore.kernel.org/all/CAKwvOd=3Jrzju++=Ve61=ZdeshxUM=K3-bGMNREnGOQgNw=aag@mail.gmail.com/ > Link: https://lore.kernel.org/all/20240418201705.3673200-2-ardb+git@google.com/ > > Cc: Arnd Bergmann > Cc: Eric Biederman > Cc: kexec@lists.infradead.org > Cc: Nathan Chancellor > Cc: Nick Desaulniers > Cc: Kees Cook > Cc: Bill Wendling > Cc: Justin Stitt > Cc: Masahiro Yamada > > Ard Biesheuvel (9): > x86/purgatory: Drop function entry padding from purgatory > x86/purgatory: Simplify stack handling > x86/purgatory: Drop pointless GDT switch > x86/purgatory: Avoid absolute reference to GDT > x86/purgatory: Simplify GDT and drop data segment > kexec: Add support for fully linked purgatory executables > x86/purgatory: Use fully linked PIE ELF executable > x86/purgatory: Simplify references to regs array > kexec: Drop support for partially linked purgatory executables > > arch/x86/include/asm/kexec.h | 8 - > arch/x86/kernel/kexec-bzimage64.c | 8 - > arch/x86/kernel/machine_kexec_64.c | 127 ---------- > arch/x86/purgatory/Makefile | 17 +- > arch/x86/purgatory/entry64.S | 96 ++++---- > arch/x86/purgatory/setup-x86_64.S | 31 +-- > arch/x86/purgatory/stack.S | 18 -- > include/asm-generic/purgatory.lds | 34 +++ > kernel/kexec_file.c | 255 +++----------------- > 9 files changed, 125 insertions(+), 469 deletions(-) > delete mode 100644 arch/x86/purgatory/stack.S > create mode 100644 include/asm-generic/purgatory.lds Eric