Re: [PATCH v16 1/3] fs: Add trusted_for(2) syscall implementation and related sysctl

["Mickaël Salaün" <mic@digikod.net>]

On 13/11/2021 20:56, Alejandro Colomar (man-pages) wrote:
> Hi Mickaël,
> 
> On 11/13/21 14:02, Mickaël Salaün wrote:
>>> TL;DR:
>>>
>>> ISO C specifies that for the following code:
>>>
>>>      enum foo {BAR};
>>>
>>>      enum foo foobar;
>>>
>>> typeof(foo)    shall be int
>>> typeof(foobar) is implementation-defined
>>
>> I tested with some version of GCC (from 4.9 to 11) and clang (10 and 11)
>> with different optimizations and the related sizes are at least the same
>> as for the int type.
> 
> GCC has -fshort-enums to make enum types be as short as possible.  I
> expected -Os to turn this on, since it saves space, but it doesn't.
> 
> Still, not relying on enum == int is better, IMO.
> 
>>
>>>
>>> Since foobar = BAR; assigns an int, the best thing to do to avoid
>>> implementation-defined behavior, is to declare foobar as int too.
>>
>> OK, so it should be enough to change the syscall argument type from enum
>> trusted_for_usage to int, but we can keep the UAPI with the enum (i.e.
>> we don't need to change the value to #define TRUSTED_FOR_EXECUTION 1)
>> right?
> 
> Correct.  The enumerations are guaranteed to be int (except in case of
> UB, see below), so they'll be (almost) the same as a #define after the
> preprocessor.

Thanks for the detailed explanation! I'll send a new patch taking into
account your suggestion.

> 
> 
> If you do
> 
> enum foo {
>     FOO = 1L << INT_WIDTH
> };
> 
> since that doesn't fit in either int or unsigned int,
> it is Undefined Behavior,
> and here GCC decides to use long for FOO.
> 
> +++++++++ UB example ++++++++++++++
> 
> $ cat foo.c
>     #include <limits.h>
>     #include <stdio.h>
> 
> 
>     enum foo {
>         FOO = 1L << UINT_WIDTH
>     };
> 
>     int main(void)
>     {
>         printf("\tsizeof(enum foo) = %zu\n", sizeof(enum foo));
>         printf("\tsizeof(FOO)      = %zu\n", sizeof(FOO));
>     }
> 
> $ cc foo.c -Wall -Wextra -Werror -Wpedantic -pedantic-errors -std=c2x
> foo.c:6:23: error: ISO C restricts enumerator values to range of 'int'
> [-Wpedantic]
>     6 |                 FOO = 1L << UINT_WIDTH
>       |                       ^~
> $ cc foo.c -Wall -Wextra -Werror -std=c2x
> $ ./a.out
>     sizeof(enum foo) = 8
>     sizeof(FOO)      = 8
> 
> +++++++++++++ -fshort-enums example +++++++++++++++
> 
> $ cat foo.c
>     #include <stdio.h>
> 
> 
>     enum foo {
>         FOO = 1
>     };
> 
>     int main(void)
>     {
>         printf("\tsizeof(enum foo) = %zu\n", sizeof(enum foo));
>         printf("\tsizeof(FOO)      = %zu\n", sizeof(FOO));
>     }
> 
> $ cc foo.c -Wall -Wextra -Werror -Wpedantic -pedantic-errors -fshort-enums
> $ ./a.out
>     sizeof(enum foo) = 1
>     sizeof(FOO)      = 4
> 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 
> Cheers,
> Alex
> 
> 
>>
>>>
>>>
>>>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>>>> index 528a478dbda8..c535e0e43cc8 100644
>>>> --- a/include/linux/syscalls.h
>>>> +++ b/include/linux/syscalls.h
>>>> @@ -462,6 +463,7 @@ asmlinkage long sys_fallocate(int fd, int mode,
>>>> loff_t offset, loff_t len);
>>>>    asmlinkage long sys_faccessat(int dfd, const char __user *filename,
>>>> int mode);
>>>>    asmlinkage long sys_faccessat2(int dfd, const char __user *filename,
>>>> int mode,
>>>>                       int flags);
>>>> +asmlinkage long sys_trusted_for(int fd, enum trusted_for_usage usage,
>>>> u32 flags);
>>>
>>> Same here.
>>>
>>>>    asmlinkage long sys_chdir(const char __user *filename);
>>>>    asmlinkage long sys_fchdir(unsigned int fd);
>>>>    asmlinkage long sys_chroot(const char __user *filename);
>>>
>>> Thanks,
>>> Alex
>>>
>>>
>