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Date:   Wed, 21 Jun 2023 13:08:52 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Evgeniy Baskov <baskov@ispras.ru>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexey Khoroshilov <khoroshilov@ispras.ru>,
        Peter Jones <pjones@redhat.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Dave Young <dyoung@redhat.com>,
        Mario Limonciello <mario.limonciello@amd.com>,
        Kees Cook <keescook@chromium.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>
Subject: Re: [PATCH v5 05/20] x86/decompressor: Use proper sequence to take
 the address of the GOT
Message-ID: <20230621110852.GAZJLaRAuGBCc5R4yb@fat_crate.local>
References: <20230607072342.4054036-1-ardb@kernel.org>
 <20230607072342.4054036-6-ardb@kernel.org>
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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