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

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

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.


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
>>
>>

-- 
Alejandro Colomar
Linux man-pages comaintainer; http://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/