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Re: [Gluster-devel] glfs_readdir_r is painful
From: |
Eric Blake |
Subject: |
Re: [Gluster-devel] glfs_readdir_r is painful |
Date: |
Wed, 30 Oct 2013 12:05:01 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.0 |
On 10/30/2013 11:18 AM, Eric Blake wrote:
> The only safe way to use readdir_r is to know the maximum d_name that
> can possibly be returned, but there is no glfs_fpathconf() for
> determining that information. Your example usage of glfs_readdir_r()
> suggests that 512 bytes is large enough:
> https://forge.gluster.org/glusterfs-core/glusterfs/blobs/f44ada6cd9bcc5ab98ca66bedde4fe23dd1c3f05/api/examples/glfsxmp.c
> but I don't know if that is true.
Okay, after a bit more investigation, I see:
gf_dirent_to_dirent (gf_dirent_t *gf_dirent, struct dirent *dirent)
{
dirent->d_ino = gf_dirent->d_ino;
#ifdef _DIRENT_HAVE_D_OFF
dirent->d_off = gf_dirent->d_off;
#endif
#ifdef _DIRENT_HAVE_D_TYPE
dirent->d_type = gf_dirent->d_type;
#endif
#ifdef _DIRENT_HAVE_D_NAMLEN
dirent->d_namlen = strlen (gf_dirent->d_name);
#endif
strncpy (dirent->d_name, gf_dirent->d_name, 256);
}
I also discovered that 'getconf NAME_MAX /path/to/xfs/mount' is 255, so
it looks like you got lucky (although strncpy is generally unsafe
because it fails to write a NUL terminator if you truncate the string,
it looks like you are guaranteed by XFS to never have a string that
needs truncation).
> You _do_ have the advantage that
> since every brick backing a glusterfs volume is using an xfs file
> system, then you only have to worry about the NAME_MAX of xfs - but I
> don't know that value off the top of my head.
Again, my research shows it is 255.
> Can you please let me
> know how big I should make my struct dirent to avoid buffer overflow,
> and properly document this in <glusterfs/api/glfs.h>? Furthermore, can
> you please provide a much saner glfs_readdir() so I don't have to worry
> about contortions of using a broken-by-design function?
These requests are still in force.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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