linux-kernel - Re: [PATCH v5 05/20] x86/decompressor: Use proper sequence to take the address of the GOT

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Date:   Wed, 21 Jun 2023 13:08:52 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Evgeniy Baskov <baskov@...ras.ru>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>,
        Kees Cook <keescook@...omium.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH v5 05/20] x86/decompressor: Use proper sequence to take
 the address of the GOT

On Wed, Jun 07, 2023 at 09:23:27AM +0200, Ard Biesheuvel wrote:
> The 32-bit decompressor does not actually use a global offset table
> (GOT), but as is common for 32-bit position independent code, it uses
> the magic symbol _GLOBAL_OFFSET_TABLE_ as an anchor from which to derive
> the actual runtime addresses of other symbols, using special @GOTOFF
> symbol references that are resolved at link time, and populated with the
> distance between the address of the magic _GLOBAL_OFFSET_TABLE_ anchor
> and the address of the symbol in question.
> 
> This means _GLOBAL_OFFSET_TABLE_ is the only symbol whose actual runtime
> address needs to be determined explicitly, which is one of the first
> things that happens in startup_32. However, it does so by taking the
> absolute address via the immediate field of an ADD instruction (plus a
> small offset), which seems to defeat the point.
> 
> Fortunately, the assembler knows that _GLOBAL_OFFSET_TABLE_ is magic,
> and emits a special relative relocation instead, and so the resulting

Which special relocation do you mean?

This guy:

Relocation section '.rel.head.text' at offset 0x3a0 contains 12 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00000010  00000d0a R_386_GOTPC       00000000   _GLOBAL_OFFSET_TABLE_

?

In any case, this thing came from

a2c4fc4d4e2c ("x86/boot: Remove run-time relocations from .head.text code")

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette