Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

[Kees Cook]

On Fri, Mar 9, 2018 at 10:10 PM, Miguel Ojeda
<miguel.ojeda.sandonis@xxxxxxxxx> wrote:
> On Sat, Mar 10, 2018 at 4:11 AM, Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
>> On 03/09/2018 04:07 PM, Andrew Morton wrote:
>>> On Fri, 9 Mar 2018 12:05:36 -0800 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>
>>>> When max() is used in stack array size calculations from literal values
>>>> (e.g. "char foo[max(sizeof(struct1), sizeof(struct2))]", the compiler
>>>> thinks this is a dynamic calculation due to the single-eval logic, which
>>>> is not needed in the literal case. This change removes several accidental
>>>> stack VLAs from an x86 allmodconfig build:
>>>>
>>>> $ diff -u before.txt after.txt | grep ^-
>>>> -drivers/input/touchscreen/cyttsp4_core.c:871:2: warning: ISO C90 forbids variable length array âidsâ [-Wvla]
>>>> -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ânamebufâ [-Wvla]
>>>> -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array âsymâ [-Wvla]
>>>> -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array âbuffâ [-Wvla]
>>>> -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array âbuffâ [-Wvla]
>>>> -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array âbuff64â [-Wvla]
>>>>
>>>> Based on an earlier patch from Josh Poimboeuf.
>>>
>>> v1, v2 and v3 of this patch all fail with gcc-4.4.4:
>>>
>>> ./include/linux/jiffies.h: In function 'jiffies_delta_to_clock_t':
>>> ./include/linux/jiffies.h:444: error: first argument to '__builtin_choose_expr' not a constant
>>
>>
>> I'm seeing that problem with
>>> gcc --version
>> gcc (SUSE Linux) 4.8.5
>
> Same here, 4.8.5 fails. gcc 5.4.1 seems to work. I compiled a minimal
> 5.1.0 and it seems to work as well.

And sparse freaks out too:

   drivers/net/ethernet/via/via-velocity.c:97:26: sparse: incorrect
type in initializer (different address spaces) @@    expected void
*addr @@    got struct mac_regs [noderef] <avoid *addr @@
   drivers/net/ethernet/via/via-velocity.c:97:26:    expected void *addr
   drivers/net/ethernet/via/via-velocity.c:97:26:    got struct
mac_regs [noderef] <asn:2>*mac_regs
   drivers/net/ethernet/via/via-velocity.c:100:49: sparse: incorrect
type in argument 2 (different base types) @@    expected restricted
pci_power_t [usertype] state @@    got _t [usertype] state @@

Alright, I'm giving up on fixing max(). I'll go back to STACK_MAX() or
some other name for the simple macro. Bleh.

-Kees

-- 
Kees Cook
Pixel Security