[PATCH] Fix readdir_r with long file names

[Michael Kerrisk (man-pages)]

Ping on this man page text. Anyone have an opinion for or against?

Thanks,

Michael


On 03/02/2016 11:39 AM, Michael Kerrisk (man-pages) wrote:
> On 03/02/2016 12:44 AM, Florian Weimer wrote:
>> On 03/02/2016 12:25 AM, Paul Eggert wrote:
>>
>>>> And at the cost of
>>>> changing sizeof (struct dirent), which can't be a good thing.
>>>
>>> Any program that depends on sizeof (struct dirent) is broken already, so
>>> this isn't that worrisome.
>>
>> Just to be clear, you looked at the wrong struct dirent definition for
>> GNU/Linux, there is a sysdeps override.
>>
>> Right now, most programs relying on sizeof (struct dirent) work well in
>> almost all cases.  We really don't want to break that.  There appears to
>> be an overlap between these programs and users of readdir_r, so once we
>> remove that from the API, we should have better story for struct dirent
>> declarators as well.
> 
> So, it seems like much more could be said about this in documentation.
> How about the following text for the man page?
> 
>    DESCRIPTION
> 
>        [...]
> 
>        In the glibc implementation, the dirent structure is defined as
>        follows:
> 
>            struct dirent {
>                ino_t          d_ino;       /* Inode number */
>                off_t          d_off;       /* Not an offset; see below */
>                unsigned short d_reclen;    /* Length of this record */
>                unsigned char  d_type;      /* Type of file; not supported
>                                               by all filesystem types */
>                char           d_name[256]; /* Null-terminated filename */
>            };
> 
>        [...]
>    NOTES
>      The d_name field
>        The  dirent  structure definition shown above is taken from the
>        glibc headers, and shows the d_name field with a fixed size.
> 
>        Warning: applications should avoid any dependence on  the  size
>        of the dname field.  POSIX defines it as char d_name[], a char‐
>        acter array of unspecified size, with at most NAME_MAX  charac‐
>        ters preceding the terminating null byte ('\0').
> 
>        POSIX.1  explicitly notes that this field should not be used as
>        an  lvalue.   The  standard  also  notes  that   the   use   of
>        sizeof(d_name)  (and  by  implication sizeof(struct dirent)) is
>        incorrect; use strlen(d_name) instead.  (On some systems,  this
>        field is defined as char d_name[1]!)
> 
>        Note that while the call
> 
>            fpathconf(fd, _PC_NAME_MAX)
> 
>        returns the value 255 for most filesystems, on some filesystems
>        (e.g., CIFS, Windows SMB servers), the null-terminated filename
>        that is (correctly) returned in d_name can actually exceed this
>        size.  (In such cases, the d_reclen field will contain a  value
>        that  exceeds  the  size  of  the  glibc dirent structure shown
>        above.)
> 
> Cheers,
> 
> Michael
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/