Messages in this thread Patch in this message | | | Date | Sat, 13 Nov 2021 20:56:09 +0100 | Subject | Re: [PATCH v16 1/3] fs: Add trusted_for(2) syscall implementation and related sysctl | From | "Alejandro Colomar (man-pages)" <> |
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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/
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