[PATCH v2 2/4] Add fchmodat4(), a new syscall

Palmer Dabbelt palmer at sifive.com
Wed Jul 17 12:12:31 AEST 2019


On Tue, 16 Jul 2019 18:48:02 PDT (-0700), viro at zeniv.linux.org.uk wrote:
> On Tue, Jul 16, 2019 at 06:27:17PM -0700, Palmer Dabbelt wrote:
>
>> -int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
>> +int do_fchmodat4(int dfd, const char __user *filename, umode_t mode, int flags)
>>  {
>>  	struct path path;
>>  	int error;
>> -	unsigned int lookup_flags = LOOKUP_FOLLOW;
>> +	unsigned int lookup_flags;
>> +
>> +	if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))
>> +		return -EINVAL;
>> +
>> +	lookup_flags = flags & AT_SYMLINK_NOFOLLOW ? 0 : LOOKUP_FOLLOW;
>> +
>
> 	Why not do that in sys_fchmodat4() itself, passing lookup_flags to
> do_fchmodat() and updating old callers to pass it 0 as extra argument?

Ya, that seems better -- passing LOOKUP_FOLLOW instead of 0, to keep the
behavior the same.  That way I could avoid the overhead of these checks for the
old syscalls, as we know they're not necessary.

I'll replace this patch with the following for a v3

    diff --git a/fs/open.c b/fs/open.c
    index b5b80469b93d..a5f99408af11 100644
    --- a/fs/open.c
    +++ b/fs/open.c
    @@ -569,11 +569,12 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
            return ksys_fchmod(fd, mode);
     }
    
    -int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
    +int do_fchmodat4(int dfd, const char __user *filename, umode_t mode,
    +                int lookup_flags)
     {
            struct path path;
            int error;
    -       unsigned int lookup_flags = LOOKUP_FOLLOW;
    +
     retry:
            error = user_path_at(dfd, filename, lookup_flags, &path);
            if (!error) {
    @@ -587,15 +588,28 @@ int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
            return error;
     }
    
    +SYSCALL_DEFINE4(fchmodat4, int, dfd, const char __user *, filename,
    +               umode_t, mode, int, flags)
    +{
    +       unsigned int lookup_flags;
    +
    +       if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))
    +               return -EINVAL;
    +
    +       lookup_flags = flags & AT_SYMLINK_NOFOLLOW ? 0 : LOOKUP_FOLLOW;
    +
    +       return do_fchmodat4(dfd, filename, mode, lookup_flags);
    +}
    +
     SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename,
                    umode_t, mode)
     {
    -       return do_fchmodat(dfd, filename, mode);
    +       return do_fchmodat4(dfd, filename, mode, LOOKUP_FOLLOW);
     }
    
     SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
     {
    -       return do_fchmodat(AT_FDCWD, filename, mode);
    +       return do_fchmodat4(AT_FDCWD, filename, mode, LOOKUP_FOLLOW);
     }
    
     static int chown_common(const struct path *path, uid_t user, gid_t group)
    diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
    index e1c20f1d0525..6676b1cc5485 100644
    --- a/include/linux/syscalls.h
    +++ b/include/linux/syscalls.h
    @@ -81,6 +81,7 @@ struct io_uring_params;
     #include <linux/quota.h>
     #include <linux/key.h>
     #include <linux/personality.h>
    +#include <linux/namei.h>
     #include <trace/syscall.h>
    
     #ifdef CONFIG_ARCH_HAS_SYSCALL_WRAPPER
    @@ -433,6 +434,8 @@ asmlinkage long sys_chroot(const char __user *filename);
     asmlinkage long sys_fchmod(unsigned int fd, umode_t mode);
     asmlinkage long sys_fchmodat(int dfd, const char __user *filename,
                                 umode_t mode);
    +asmlinkage long sys_fchmodat4(int dfd, const char __user *filename,
    +                            umode_t mode, int flags);
     asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
                                 gid_t group, int flag);
     asmlinkage long sys_fchown(unsigned int fd, uid_t user, gid_t group);
    @@ -1320,11 +1323,12 @@ static inline long ksys_link(const char __user *oldname,
            return do_linkat(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
     }
    
    -extern int do_fchmodat(int dfd, const char __user *filename, umode_t mode);
    +extern int do_fchmodat4(int dfd, const char __user *filename, umode_t mode,
    +                       int flags);
    
     static inline int ksys_chmod(const char __user *filename, umode_t mode)
     {
    -       return do_fchmodat(AT_FDCWD, filename, mode);
    +       return do_fchmodat4(AT_FDCWD, filename, mode, LOOKUP_FOLLOW);
     }
    
     extern long do_faccessat(int dfd, const char __user *filename, int mode);


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