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Date:   Sun, 17 Sep 2023 12:26:03 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Greg Ungerer <gerg@kernel.org>
Cc:     linux-arm@lists.infradead.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        eescook@chromium.org, ebiederm@xmission.com, brauner@kernel.org,
        viro@zeniv.linux.org.uk
Subject: Re: [PATCH v2] fs: binfmt_elf_efpic: fix personality for ELF-FDPIC
Message-Id: <20230917122603.5466b1fc6d05ea9e9edee340@linux-foundation.org>
In-Reply-To: <20230907011808.2985083-1-gerg@kernel.org>
References: <20230907011808.2985083-1-gerg@kernel.org>
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On Thu,  7 Sep 2023 11:18:08 +1000 Greg Ungerer <gerg@kernel.org> wrote:

> The elf-fdpic loader hard sets the process personality to either
> PER_LINUX_FDPIC for true elf-fdpic binaries or to PER_LINUX for
> normal ELF binaries (in this case they would be constant displacement
> compiled with -pie for example). The problem with that is that it
> will lose any other bits that may be in the ELF header personality
> (such as the "bug emulation" bits).
> 
> On the ARM architecture the ADDR_LIMIT_32BIT flag is used to signify
> a normal 32bit binary - as opposed to a legacy 26bit address binary.
> This matters since start_thread() will set the ARM CPSR register as
> required based on this flag. If the elf-fdpic loader loses this bit
> the process will be mis-configured and crash out pretty quickly.
> 
> Modify elf-fdpic loader personality setting so that it preserves the
> upper three bytes by using the SET_PERSONALITY macro to set it. This
> macro in the generic case sets PER_LINUX and preserves the upper bytes.
> Architectures can override this for their specific use case, and ARM
> does exactly this.
> 
> The problem shows up quite easily running under qemu using the ARM
> architecture, but not necessarily on all types of real ARM hardware.
> If the underlying ARM processor does not support the legacy 26-bit
> addressing mode then everything will work as expected.

I'm thinking

Fixes: 1bde925d23547 ("fs/binfmt_elf_fdpic.c: provide NOMMU loader for regular ELF binaries")
Cc: <stable@vger.kernel.org>

?