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Re: [libvirt] [PATCH 1/2] util: Don't fail virGetUserIDByName when user not found



On 12/06/12 10:59, Peter Krempa wrote:
On 12/06/12 10:25, Osier Yang wrote:
On 2012年12月05日 20:03, Christophe Fergeau wrote:
virGetUserIDByName is documented as returning 1 if the username
cannot be found. getpwnam_r is documented as returning:
« 0 or ENOENT or ESRCH or EBADF or EPERM or ...  The given name
or uid was not found. »
  and that:
« The formulation given above under "RETURN VALUE" is from POSIX.1-2001.
It  does  not  call  "not  found"  an error, hence does not specify what
value errno might have in this situation.  But that makes it
impossible to
recognize errors.  One might argue that according to POSIX errno
should be
left unchanged if an entry is not found.  Experiments on various
UNIX-like
systems shows that lots of different values occur in this situation: 0,
ENOENT, EBADF, ESRCH, EWOULDBLOCK, EPERM and probably others. »

virGetUserIDByName returns an error when the return value of getpwnam_r
is non-0. However on my RHEL system, getpwnam_r returns ENOENT when the
requested user cannot be found, which then causes virGetUserID not
to behave as documented (it returns an error instead of falling back
to parsing the passed-in value as an uid).

This commit makes virGetUserIDByName ignore the various values listed
in getpwnam_r manpage and return a 'user not found' result in such
cases.
---
  src/util/util.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/util/util.c b/src/util/util.c
index 2fd0f2c..df1af7e 100644
--- a/src/util/util.c
+++ b/src/util/util.c
@@ -2530,7 +2530,8 @@ virGetUserIDByName(const char *name, uid_t *uid)
          }
      }

-    if (rc != 0) {
+    if ((rc != 0)&&  (rc != ENOENT)&&  (rc != ESRCH)
+&&  (rc != EBADF)&&  (rc != EPERM)) {

Correct indention should be:

     if ((rc != 0) && (rc != ENOENT) && (rc != ESRCH) &&
         (rc != EBADF) && (rc != EPERM))

When I was changing the code the first time, I got rid of this as the
docs for getpwnam_r seemed clear enough to me that this shouldn't be
needed. But I was wrong and I'd rather not reintroduce this ugly and
nondeterministic error checking.


Per the POSIX standard, even these errors are not enough,
should we give up using getpwnam_r/getgrnam_r, and create
helpers using getpwent/getgrent to do the lookup work
ourselves?

Hmm, those look fine to me. I didn't do my research properly when
changing the code the first time.


<...>
        The getpwent() function returns a pointer to a passwd structure,

getpwent() isn't reentrant, and so isn't the supposedly reentrant version getpwent_r():

from the manpage:
NOTES
The function getpwent_r() is not really reentrant since it shares the reading position in the stream with all other threads.

I will try to restructure the control flow so that if there's a fallback method (numeric parsing of the input string succeeds) errors to get the entry won't be fatal, but I'd like to avoid whitelisting of the return variables as the definition of them is vague.


or NULL if there are no more entries or an error occurs.  If an
        error occurs, errno is set appropriately.  If one wants to check
errno after the call, it should be set to zero before the call.
</...>

<...>
ERRORS
        EINTR  A signal was caught.

        EIO    I/O error.

        EMFILE The maximum number (OPEN_MAX) of files was open already
in the calling process.

        ENFILE The maximum number of files was open already in the
system.

        ENOMEM Insufficient memory to allocate passwd structure.

        ERANGE Insufficient buffer space supplied.
</...>

I think it's pretty easy to distiguish NOT FOUND and other error
then.

That's true. I'll look into this and check if it's feasible for us to do
in libvirt.


Regards,
Osier

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