RE: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter

["Maciej W. Rozycki" <macro@mips.com>]

Hi Miodrag,

> When kernel is detecting the type of mapping it should apply :
> 
> fs/binfmt_elf.c:
> ...
>       if (elf_read_implies_exec(loc->elf_ex, executable_stack))
>               current->personality |= READ_IMPLIES_EXEC;
> ...
> 
> this effectively calls mips_elf_read_implies_exec() which performs a check:
> ...
>       if (!cpu_has_rixi) {
>               /* The CPU doesn't support non-executable memory */
>               return 1;
>       }
> 
>       return 0;
> }
> 
> This will in turn make stack & heap executable on processors without 
> RIXI, which are practically all processors with MIPS ISA R < 6.
> 
> We would like to have an option to override this and force 
> non-executable mappings for such systems.

 Of course you can't force a non-executable mapping with a system where 
all valid pages are executable, as David has already noted.  Did you mean 
the other condition, that is:

        if (exstack != EXSTACK_DISABLE_X) {
                /* The binary doesn't request a non-executable stack */
                return 1;
        }

?  In which case you do want to respect the lack of the RIXI feature, 
i.e.:

int mips_elf_read_implies_exec(void *elf_ex, int exstack)
{
        if (!cpu_has_rixi) {
                /* The CPU doesn't support non-executable memory */
                return 1;
        }

        switch (nonxstack) {
        case EXSTACK_DISABLE_X:
                return 0;
        case EXSTACK_ENABLE_X:
                return 1;
        default:
                break;
        }

        if (exstack != EXSTACK_DISABLE_X) {
                /* The binary doesn't request a non-executable stack */
                return 1;
        }

        return 0;
}

(I'd replace `break' with `return exstack != EXSTACK_DISABLE_X' and 
discard the code that follows, but that can be a separate optimisation).

 What problem are you trying to solve anyway?  Is it not something that 
can be handled with the `execstack' utility?

 NB as someone has observed with programs that do not request a 
non-executable stack we actually propagate the execute permission to all 
data pages.  Is it not something we would want to handle differently?

  Maciej