From: Fangrui Song <maskray@google.com>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, Nick Desaulniers <ndesaulniers@google.com>,
Dmitry Golovin <dima@golovin.in>,
clang-built-linux@googlegroups.com,
Ard Biesheuvel <ardb@kernel.org>,
Masahiro Yamada <masahiroy@kernel.org>,
Daniel Kiper <daniel.kiper@oracle.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] x86/boot: Remove runtime relocations from .head.text code
Date: Sun, 24 May 2020 17:55:11 -0700 [thread overview]
Message-ID: <20200525005511.2aar7gfnklbnzj74@google.com> (raw)
In-Reply-To: <20200524234402.GB280334@rani.riverdale.lan>
On 2020-05-24, Arvind Sankar wrote:
>On Sun, May 24, 2020 at 03:53:59PM -0700, Fangrui Song wrote:
>> On 2020-05-24, Arvind Sankar wrote:
>> >The assembly code in head_{32,64}.S, while meant to be
>> >position-independent, generates run-time relocations because it uses
>> >instructions such as
>> > leal gdt(%edx), %eax
>> >which make the assembler and linker think that the code is using %edx as
>> >an index into gdt, and hence gdt needs to be relocated to its run-time
>> >address.
>> >
>> >With the BFD linker, this generates a warning during the build:
>> > LD arch/x86/boot/compressed/vmlinux
>> >ld: arch/x86/boot/compressed/head_32.o: warning: relocation in read-only section `.head.text'
>> >ld: warning: creating a DT_TEXTREL in object
>>
>> Interesting. How does the build generate a warning by default?
>> Do you use Gentoo Linux which appears to ship a --warn-shared-textrel
>> enabled-by-default patch? (https://bugs.gentoo.org/700488)
>
>Ah, yes I am using gentoo. I didn't realize that was a distro
>modification.
>
>> >+
>> >+/*
>> >+ * This macro gives the link address of X. It's the same as X, since startup_32
>> >+ * has link address 0, but defining it this way tells the assembler/linker that
>> >+ * we want the link address, and not the run-time address of X. This prevents
>> >+ * the linker from creating a run-time relocation entry for this reference.
>> >+ * The macro should be used as a displacement with a base register containing
>> >+ * the run-time address of startup_32 [i.e. la(X)(%reg)], or as an
>> >+ * immediate [$ la(X)].
>> >+ *
>> >+ * This macro can only be used from within the .head.text section, since the
>> >+ * expression requires startup_32 to be in the same section as the code being
>> >+ * assembled.
>> >+ */
>> >+#define la(X) ((X) - startup_32)
>> >+
>>
>> IIRC, %ebp contains the address of startup_32. la(X) references X
>> relative to startup_32. The fixup (in GNU as and clang integrated
>> assembler's term) is a constant which is resolved by the assembler.
>>
>> There is no R_386_32 or R_386_PC32 for the linker to resolve.
>
>This is incorrect (or maybe I'm not understanding you correctly). X is a
>symbol whose final location relative to startup_32 is in most cases not
>known to the assembler (there are a couple of cases where X is a label
>within .head.text which do get completely resolved by the assembler).
>
>For example, taking the instruction loading the gdt address, this is
>what we get from the assembler:
> lea la(gdt)(%ebp), %eax
>becomes in the object file:
> 11: 8d 85 00 00 00 00 lea 0x0(%ebp),%eax
> 13: R_X86_64_PC32 .data+0x23
>or a cleaner example using a global symbol:
> subl la(image_offset)(%ebp), %ebx
>becomes
> 41: 2b 9d 00 00 00 00 sub 0x0(%ebp),%ebx
> 43: R_X86_64_PC32 image_offset+0x43
>
>So in general you get PC32 relocations, with the addend being set by the
>assembler to .-startup_32, modulo the adjustment for where within the
>instruction the displacement needs to be. These relocations are resolved
>by the static linker to produce constants in the final executable.
>
You are right. I am not familiar with the code and only looked at 1b.
Just preprocessed head_64.S and verified many target symbols are in
.data and .pgtable The assembler converts an expression `foo - symbol_defined_in_same_section`
to be `foo - . + offset` which can be encoded as an R_X86_64_PC32 (or
resolved the fixup if it is a constant, e.g. `1b - startup_32`)
>>
>> Not very sure stating that "since startup_32 has link address 0" is very
>> appropriate here (probably because I did't see the term "link address"
>> before). If my understanding above is correct, I think you can just
>> reword the comment to express that X is referenced relative to
>> startup_32, which is stored in %ebp.
>>
>
>Yeah, the more standard term is virtual address/VMA, but that sounds
>confusing to me with PIE code since the _actual_ virtual address at
>which this code is going to run isn't 0, that's just the address assumed
>for linking. I can reword it to avoid referencing "link address" but
>then it's not obvious why the macro is named "la" :) I'm open to
>suggestions on a better name, I could use offset but that's a bit
>long-winded. I could just use vma() if nobody else finds it confusing.
>
>Thanks.
With your approach, the important property is that "the distance between
startup_32 and the target symbol is a constant", not that "startup_32
has link address 0". `ra`, `rva`, `rvma` or any other term which expresses "relative"
should work. Hope someone can come up with a good suggestion:)
next prev parent reply other threads:[~2020-05-25 0:55 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-01 8:42 [PATCH] x86/boot: allow a relocatable kernel to be linked with lld Dmitry Golovin
2020-05-02 3:43 ` Nathan Chancellor
2020-05-15 18:50 ` Borislav Petkov
[not found] ` <602331589572661@mail.yandex.ru>
2020-05-17 19:44 ` Fangrui Song
2020-05-17 20:25 ` Arvind Sankar
2020-05-18 19:10 ` Nick Desaulniers
2020-05-24 21:28 ` [PATCH 0/4] x86/boot: Remove runtime relocations from compressed kernel Arvind Sankar
2020-05-25 7:10 ` Ard Biesheuvel
2020-05-25 22:59 ` [PATCH v2 " Arvind Sankar
2020-05-26 12:29 ` Sedat Dilek
2020-05-26 12:30 ` Ard Biesheuvel
2020-05-26 12:33 ` Sedat Dilek
2020-05-26 12:44 ` Sedat Dilek
2020-05-26 14:47 ` Arvind Sankar
2020-05-26 14:50 ` Sedat Dilek
2020-05-26 15:36 ` Arvind Sankar
2020-05-26 15:38 ` Sedat Dilek
2020-05-27 6:26 ` Sedat Dilek
2020-05-26 14:48 ` Sedat Dilek
2020-05-26 14:55 ` Sedat Dilek
2020-05-26 15:07 ` Sedat Dilek
2020-05-26 15:31 ` Arvind Sankar
2020-05-27 6:24 ` Sedat Dilek
2020-05-26 16:18 ` Sedat Dilek
2020-05-25 22:59 ` [PATCH v2 1/4] x86/boot: Add .text.* to setup.ld Arvind Sankar
2020-05-25 22:59 ` [PATCH v2 2/4] x86/boot: Remove run-time relocations from .head.text code Arvind Sankar
2020-05-25 22:59 ` [PATCH v2 3/4] x86/boot: Remove runtime relocations from head_{32,64}.S Arvind Sankar
2020-05-25 22:59 ` [PATCH v2 4/4] x86/boot: Check that there are no runtime relocations Arvind Sankar
2020-05-26 6:11 ` Ard Biesheuvel
2020-05-26 15:16 ` Arvind Sankar
2020-05-26 17:13 ` Fangrui Song
2020-05-26 19:14 ` Arvind Sankar
2020-08-06 11:19 ` Andy Shevchenko
2020-08-06 16:12 ` Arvind Sankar
2020-05-26 0:37 ` [PATCH 0/4] x86/boot: Remove runtime relocations from compressed kernel Fangrui Song
2020-05-24 21:28 ` [PATCH 1/4] x86/boot: Add .text.startup to setup.ld Arvind Sankar
2020-05-24 22:13 ` Fangrui Song
2020-05-24 23:00 ` Arvind Sankar
2020-05-24 23:49 ` Fangrui Song
2020-05-24 22:48 ` Brian Gerst
2020-05-24 21:28 ` [PATCH 2/4] x86/boot: Remove runtime relocations from .head.text code Arvind Sankar
2020-05-24 22:53 ` Fangrui Song
2020-05-24 23:44 ` Arvind Sankar
2020-05-25 0:55 ` Fangrui Song [this message]
2020-05-24 21:28 ` [PATCH 3/4] x86/boot: Remove runtime relocations from head_{32,64}.S Arvind Sankar
2020-05-24 23:22 ` Fangrui Song
2020-05-24 23:58 ` Arvind Sankar
2020-05-24 21:28 ` [PATCH 4/4] x86/boot: Check that there are no runtime relocations Arvind Sankar
2020-05-24 23:36 ` Fangrui Song
2020-05-24 23:57 ` Arvind Sankar
2020-05-25 6:10 ` Ard Biesheuvel
2020-05-25 16:26 ` Fangrui Song
2020-05-25 19:22 ` Arvind Sankar
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