[PATCH v2] fs: binfmt_elf_efpic: fix personality for ELF-FDPIC

[Andrew Morton]

On Thu,  7 Sep 2023 11:18:08 +1000 Greg Ungerer <gerg at 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 at vger.kernel.org>

?