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Re: [Libguestfs] [PATCH 4/5] fuse: Fix read for empty files.



On 17/11/09 17:07, Richard W.M. Jones wrote:
From 3c5d63be17d281634f036f53aa32103ad4ab1c41 Mon Sep 17 00:00:00 2001
From: Richard Jones<rjones redhat com>
Date: Tue, 17 Nov 2009 17:02:45 +0000
Subject: [PATCH 4/5] fuse: Fix read for empty files.

Error handling for the guestfs_pread call was incorrect, which
meant that empty files could produce spurious error messages.
---
  fuse/guestmount.c |    8 +++++++-
  1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/fuse/guestmount.c b/fuse/guestmount.c
index 9d16cef..669bf80 100644
--- a/fuse/guestmount.c
+++ b/fuse/guestmount.c
@@ -621,8 +621,14 @@ fg_read (const char *path, char *buf, size_t size, off_t offset,
    if (size>  limit)
      size = limit;

+  /* Note the correct error handling here is tricky, because in the
+   * case where the call returns a zero-length buffer, it might return
+   * NULL.  However it won't adjust rsize along the error path, so we
+   * can set rsize to something beforehand and use that as a flag.
+   */
+  rsize = 1;
    r = guestfs_pread (g, path, size, offset,&rsize);
-  if (r == NULL)
+  if (rsize == 1&&  r == NULL)
      return error ();

    /* This should never happen, but at least it stops us overflowing
-- 1.6.5.2

I've stared at this til my head hurts, and I don't get it. Why is 1 not a valid returned size from guestfs_pread? Why is NULL not an indicator of failure?

The documentation says:

       This function returns a buffer, or NULL on error.  The size of the
       returned buffer is written to *size_r.  The caller must free the
       returned buffer after use.

If NULL isn't sufficient to indicate error, guestfs_pread needs to be fixed, not this funtion.

Matt
--
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

M:       +44 (0)7977 267231
GPG ID:  D33C3490
GPG FPR: 3733 612D 2D05 5458 8A8A 1600 3441 EA19 D33C 3490


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