Re: [PATCH] selinux: optimize major part with a kernel config in selinux_mmap_addr()

[Thomas Weißschuh]

On 2023-07-10 17:25:00+0900, Leesoo Ahn wrote:
> The major part, the conditional branch in selinux_mmap_addr() is always to be
> false so long as CONFIG_LSM_MMAP_MIN_ADDR is set to zero at compile time.
> 
> This usually happens in some linux distros, for instance Ubuntu, which
> the config is set to zero in release version. Therefore it could be a bit
> optimized with '#if <expr>' at compile time.
> 
> Signed-off-by: Leesoo Ahn <lsahn@xxxxxxxxxxxxxx>
> ---
>  security/selinux/hooks.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index d06e350fedee..a049aab6524b 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3723,11 +3723,13 @@ static int selinux_mmap_addr(unsigned long addr)
>  {
>  	int rc = 0;
>  
> +#if CONFIG_LSM_MMAP_MIN_ADDR > 0
>  	if (addr < CONFIG_LSM_MMAP_MIN_ADDR) {
>  		u32 sid = current_sid();
>  		rc = avc_has_perm(sid, sid, SECCLASS_MEMPROTECT,
>  				  MEMPROTECT__MMAP_ZERO, NULL);
>  	}
> +#endif

Shouldn't the compiler figure out on its own that "0 < 0" is always
false and optimize it all away? My gcc 13.1.1 does so.

Without your change:

$ ./scripts/bloat-o-meter  security/selinux/hooks.o-min-addr-64k security/selinux/hooks.o-min-addr-0
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-65 (-65)
Function                                     old     new   delta
selinux_mmap_addr                             81      16     -65
Total: Before=57673, After=57608, chg -0.11%

The same with your patch and also with the proposal by Paul that
redefines the whole function to "return 0".