[PATCH 3/5] exec: Remove recursion from search_binary_handler

[Rob Landley]

On 5/11/20 9:33 AM, Eric W. Biederman wrote:
> What I do see is that interp_data is just a parameter that is smuggled
> into the call of search binary handler.  And the next binary handler
> needs to be binfmt_elf for it to make much sense, as only binfmt_elf
> (and binfmt_elf_fdpic) deals with BINPRM_FLAGS_EXECFD.

The binfmt_elf_fdpic driver is separate from binfmt_elf for the same reason
ext2/ext3/ext4 used to have 3 drivers: fdpic is really just binfmt_elf with the
4 main sections (text, data, bss, rodata) able to move independently of each
other (each tracked with its own base pointer).

It's kind of -fPIE on steroids, and various security people have sniffed at it
over the years to give ASLR more degrees of freedom on with-MMU systems. Many
moons ago Rich Felker proposed teaching the fdpic loader how to load normal ELF
binaries so there's just the one loader (there's a flag in the ELF header to say
whether the sections are independent or not).

Rob