stat() broken on CRIS with _FILE_OFFSET_BITS == 64

[Peter Kjellerstedt]

[ If the mail below seems somewhat unstructured it is because
  I continued to debug while I was writing the mail and thus
  found out new facts as I carried along. ]

I was compiling an application which uses large file support for
the CRIS architecture, and it failed mysteriously. After some 
debugging I found the culprit in stat(). The attached test 
program (compiled with -O0) gives me the following output:
 
stat(): 0x000802F8
stat64(): 0x000802F8
sizeof(struct stat): 92
sizeof(struct stat64): 96
sizeof file_info: 92
file_info.st_size: 2334846631540
((struct stat64*)&file_info)->st_size: 543
magic1: 0x00000000
magic2: 0xDEADBEEF

As can be seen above both stat() and stat64() refers to the same 
function (i.e., stat64()), but struct stat and struct stat64 are
not the same. Thus the call to stat() with a struct stat argument
will lead to incorrect data and even overwritten memory (as seen 
by the magic1 variable).

I believe the problem originates in include/sys/stat.h (line 
205-224) which looks like this:

#ifndef __USE_FILE_OFFSET64
/* Get file attributes for FILE and put them in BUF.  */
extern int stat (__const char *__restrict __file,
		 struct stat *__restrict __buf) __THROW __nonnull ((1,
2));

/* Get file attributes for the file, device, pipe, or socket
   that file descriptor FD is open on and put them in BUF.  */
extern int fstat (int __fd, struct stat *__buf) __THROW __nonnull ((2));
#else
# ifdef __REDIRECT_NTH
extern int __REDIRECT_NTH (stat, (__const char *__restrict __file,
				  struct stat *__restrict __buf),
stat64)
     __nonnull ((1, 2));
extern int __REDIRECT_NTH (fstat, (int __fd, struct stat *__buf),
fstat64)
     __nonnull ((2));
# else
#  define stat stat64
#  define fstat fstat64
# endif
#endif

As can be seen above, if __USE_FILE_OFFSET64 is defined together 
with __REDIRECT_NTH then stat() will be aliased to stat64(). 
However, struct stat will _not_ be the same as struct stat64!

Then if one further looks in sysdeps/linux/common/bits/stat.h one 
notices that the declaration of struct stat contains a number of 
#ifndef __USE_FILE_OFFSET64 which seem to try to make the struct
look the same as struct stat64 when __USE_FILE_OFFSET64 is defined.
However, there is a minor difference: the two padding fields are
unsigned short int in struct stat and unsigned int in struct stat64.

The reason that this is not a problem on i386 is that the compiler
actually pads the two pad fields so they are actually four bytes.
On CRIS, however there is no reason for the compiler to do this
padding and the resulting structures are thus not the same...

The attached patch fixes this. I think it should be ok to apply 
since uClibc already provide both stat() and stat64() so applications
which call either should continue to work. And those that thought
they called stat() but actually called stat64() will continue to
be broken (on architectures with packed structures like CRIS), until 
they are recompiled when they will magically start to work...

An alternative approach would be to change the two pad fields in 
struct stat to be unsigned long. This should still generate the 
same binary object on i386, but also work for architectures with 
packed structures (after recompilation).

What I do not understand is why that padding is there in the first
place. My guess is that it is a remnant from when dev_t was two 
bytes in size, but today it is eight bytes, and that padding 
actually makes the structure fields unaligned...

So the best would have been to remove the padding altogether. 
This is of course not binary compatible with previous releases...

//Peter
 <<stat_test.c>> 
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