Re: [Qemu-devel] [PATCH] block/vvfat: Fix crash when reporting error about too many files in directory

[Thomas Huth <thuth@redhat.com>]

On 23.07.2018 16:33, Kevin Wolf wrote:
> Am 18.07.2018 um 17:28 hat Thomas Huth geschrieben:
>> When using the vvfat driver with a directory that contains too many files,
>> QEMU currently crashes. We are trying to print the wrong path variable here.
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>  block/vvfat.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/block/vvfat.c b/block/vvfat.c
>> index fc41841..6ae7458 100644
>> --- a/block/vvfat.c
>> +++ b/block/vvfat.c
>> @@ -975,8 +975,7 @@ static int init_directories(BDRVVVFATState* s,
>>          if (mapping->mode & MODE_DIRECTORY) {
>>              mapping->begin = cluster;
>>              if(read_directory(s, i)) {
>> -                error_setg(errp, "Could not read directory %s",
>> -                           mapping->path);
>> +                error_setg(errp, "Could not read directory \"%s\"", s->path);
> 
> Hm, I'm not sure if that's right. Before this patch we were printing
> the name of the subdirectory that couldn't be loaded, now it's the
> parent directory.
> 
> My test case where this difference is visible is a subdirectory with
> chmod 000.

Right.

>>                  return -1;
>>              }
>>              mapping = array_get(&(s->mapping), i);
> 
> Maybe the right solution would be moving the reloading of mapping to
> between the read_directory() call and the error path?

No, that does not work either. The problem seems to be that
read_directory() is changing the mapping->path pointer to something
invalid in between, but I've been unable to track it down where it
happens. This patch here seems to work for me, though:

diff --git a/block/vvfat.c b/block/vvfat.c
index fc41841..f2e7d50 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -973,10 +973,10 @@ static int init_directories(BDRVVVFATState* s,
         mapping = array_get(&(s->mapping), i);

         if (mapping->mode & MODE_DIRECTORY) {
+            char *path = mapping->path;
             mapping->begin = cluster;
             if(read_directory(s, i)) {
-                error_setg(errp, "Could not read directory %s",
-                           mapping->path);
+                error_setg(errp, "Could not read directory %s", path);
                 return -1;
             }
             mapping = array_get(&(s->mapping), i);

Does this look reasonable for you, too?

 Thomas