_SC_GETPW_R_SIZE_MAX undefined in sysconf.c, what is correct value?

[Christopher J. Ruwe]

On Tue, 25 Oct 2011 16:27:38 -0500
Dan Nelson <dnelson at allantgroup.com> wrote:

> In the last episode (Oct 25), Christopher J. Ruwe said:
> > On Mon, 24 Oct 2011 15:42:10 -0500
> > Dan Nelson <dnelson at allantgroup.com> wrote:
> > > In the last episode (Oct 24), Christopher J. Ruwe said:
> > > > On Sun, 23 Oct 2011 19:10:34 -0500
> > > > Dan Nelson <dnelson at allantgroup.com> wrote:
> > > > > In the last episode (Oct 23), Christopher J. Ruwe said:
> > > > > > I need to get the maximum size of an pwd-entry to determine
> > > > > > the correct buffersize for calling getpwnam_r("uname",&pwd,
> > > > > > buf, bufsize, &pwdp).  I would like to use
> > > > > > sysconf(_SC_GETPW_R_SIZE_MAX) to determine bufsize, which
> > > > > > unfornutately fails (returns -1).  Currently, I used 16384,
> > > > > > which seems to be too much, bit works for the time being.
> > > [..]
> > > > > From looking at the libc/gen/getpwent.c file, it looks like a
> > > > > maximum size might be 1MB.  The wrapper functions that convert
> > > > > getpw*_r functions into ones that simply return a pointer to
> > > > > malloced data all use the getpw() helper function, which
> > > > > starts with a 1k buffer and keeps doubling its size until the
> > > > > data fits or it hits PWD_STORAGE_MAX (1MB).  PWD_STORAGE_MAX
> > > > > is only checked within that getpw() function, though, so it's
> > > > > possible that an nss library might return an even longer
> > > > > string to a get*_r call.  It's up to you to decide what your
> > > > > own limit is :)
> > > >
> > > > Uh ... it's just that I hoped I had not to decide ;-)
> > > 
> > > The getpwnam_r function needs enough space to store the "struct
> > > passwd" itself (which has a constant size) plus the strings
> > > pointed to by pw_name, pw_class, pw_gecos, pw_dir, and pw_shell.
> > > If you have enough control over your environment that you can
> > > guarantee that the sum of those strings won't be larger than 4k,
> > > then you can just used a fixed buffer of that size.  Even 1k is
> > > probably large enough for 99.999% of all systems.  That's a
> > > really long home directory or shell path :) On the other hand,
> > > the GECOS field is theoretially free-form and could contain a lot
> > > of data.  I've never see it hold more than an office number
> > > myself, though.
> > > 
> > 
> > Thanks for your help so far. Just assuming (I am not sufficiently
> > clear about myself and my own intents) I want to be precise and am
> > afraid of guessing: Can I assume that the gecos field is an entry
> > in /etc/passwd and can therefore never exceed LINE_MAX, i.e., 2048B
> > (limits.h, line 72)?  Or, more precisely, ( 2048B - sum( lenght(all
> > fields except passwd) ) )? Would that be an acceptable limit to set
> > the getpwnam_r( ...  ) buffer to and/or would that be an acceptable
> > value to replace the following bit from sysconf.c?
> > 
> >     372	   #if _POSIX_THREAD_SAFE_FUNCTIONS > -1
> >     373 	case _SC_GETGR_R_SIZE_MAX:
> >     374 	case _SC_GETPW_R_SIZE_MAX:
> >     375 	#error "somebody needs to implement this"
> >     376    #endif
> 
> If your nsswitch.conf has "passwd: files" in it, then yes, you can
> assume that the 2048-byte limit applies.  However, if you are using
> nss_ldap, nss_mysql, nss_winbind, or some other nsswitch module that
> provides user info, that backend user system may be capable of
> returning longer strings. If you want to be able to handle any struct
> passwd that might be thrown at you, you should implement a "retry
> with doubling" loop similar to the one in
> libc/gen/getpwent.c:getpw() . 

This method has been suggested at some other sites as well ... I just had hopes it would be able to implement that in a more elegant and more concise manner.

Anyways, thank you for your kind help, cheers,
-- 
Christopher J. Ruwe
TZ GMT + 2