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Date: Sun, 14 Nov 2021 13:09:06 +0100
From: Mickaël Salaün <mic@...ikod.net>
To: "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>,
Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Aleksa Sarai <cyphar@...har.com>, Andy Lutomirski <luto@...nel.org>,
Arnd Bergmann <arnd@...db.de>, Casey Schaufler <casey@...aufler-ca.com>,
Christian Brauner <christian.brauner@...ntu.com>,
Christian Heimes <christian@...hon.org>,
Deven Bowers <deven.desai@...ux.microsoft.com>,
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Eric Chiang <ericchiang@...gle.com>, Florian Weimer <fweimer@...hat.com>,
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Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
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linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org,
Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v16 1/3] fs: Add trusted_for(2) syscall implementation and
related sysctl
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
>>>
>>>
>
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