Re: RAMFS fixes: file permissions and lseek()

[Dan Jakubiec <dan dot jakubiec at systech dot com>]

Hi Andrew,

Thanks for the review and the comments.  Your suggestions look like good 
improvements.  My responses and questions:

Andrew Lunn wrote:

> OK. But i really thing it is the application which is broken. eCos has
> no concept of filesystem security, there is no way to set permissions,
> owners, groups etc. So any application which uses this is probably
> broken.
>
>  
Well, perhaps...  But consider this:  we ran into this permissions 
problem when porting a POSIX application to eCos.  The application 
happened to be the eCos mk_romfs.c host-side utility for generating 
ROMFS images.  [our eCos environment needs the ability to generate ROMFS 
images at runtime based on our RAMFS root filesystem]

This POSIX app (like others I'm sure), checks each file to make sure it 
is readable prior to building its ROMFS image.  Without the fix, the 
POSIX app won't proceed with the ROMFS generation because it expects 
that its planned open() calls will fail.  We could hack mk_romfs to 
behave differently under eCos, but I think there is a more elegant solution.

The RAMFS fix is rather reasonable because although file/directory 
permissions can not be *set* (nor enforced), than can still be read by 
applications and therefore should reflect the reality of the file 
security.  In the eCos RAMFS filesystem:

  1. All files are readable and writable by anyone, so files should at
     minimum report a permissions mask of 666.
  2. The executable bit is debatable.  However, I don't think it hurts
     to set it so I did.  This will keep happy those ported apps which
     expect to find executable files with correct perms.  Of course,
     these files aren't truly executable.  If you think 666 is more
     logical here, then I'm open to your thinking.  777 just seemed
     more all-encompassing.
  3. For directories: all directories are readable, writable, and
     viewable by all.  So 777 seemed the most appropriate.
  4. A permission mask of 000 seems wrong on all accounts.  Because
     although it won't protect the files from being used, it will
     misrepresent their true read/write characteristics in the stat() call.

> I fixed this differently, in a more efficient way. For the default
> "SIMPLE" allocation mechanism, there was already a memzero setting the
> contents to zero. It just needed tweeking a little. For the "BLOCK"
> allocation mechanism i implemented true holes. So you can do something
> like
>
> fd = open("foobar", O_RDWR);
> lseek(fd, 1024*1024, SEEK_SET);
> write(fd, '1', 1);
>
> and it will only allocation one 256 byte block, not a megabyte.
>  
This sound like a better fix, but I had once concern.  I haven't 
familiarized myself with the BLOCK approach, so perhaps you can fill in 
the blanks here.  According to POSIX, once you write() to the file at 
the post-EOF position, all reads between the old EOF and the new write 
data must now return byte 0x00 in each unwritten location.  In other 
words, the act of the write() is supposed to increase the file size by 
the number of write bytes plus the gap size.  In other words:

  1. fd = open()
  2. write(fd, "hi", 2);      //lpos at 2
  3. lseek(fd, 4, SEEK_SET);  //lpos at 4
  4. write(fd, "you", 3);     //lpos at 7
  5. lseek(fd, 0, SEEK_SET);  //lpos at 0
  6. read(fd, buffer, 100) ==> read() returns 7, buffer contains
     "hi\x00\x00you"

I will study your improvements, but will they indeed return the 0x00 
bytes as required by POSIX?

Thanks,

--
Dan Jakubiec
Systech Corporation