Re: [patch] ext2_fill_super breakage

[Andrew Morton <akpm@zip.com.au>]

Brian Gerst wrote:
> 
> Andrew Morton wrote:
> > In 2.5.7 there is a thinko in the allocation and initialisation
> > of the fs-private superblock for ext2.  It's passing the wrong type
> > to the sizeof operator (which of course gives the wrong size)
> > when allocating and clearing the memory.
> >
> > Lesson for the day: this is one of the reasons why this idiom:
> >
> >       some_type *p;
> >
> >       p = malloc(sizeof(*p));
> >       ...
> >       memset(p, 0, sizeof(*p));
> >
> > is preferable to
> >
> >       some_type *p;
> >
> >       p = malloc(sizeof(some_type));
> >       ...
> >       memset(p, 0, sizeof(some_type));
> >
> > I checked the other filesystems.  They're OK (but idiomatically
> > impure).  I've added a couple of defensive memsets where
> > they were missing.
> 
> I'm not sure I follow you here.  Compiling this code (gcc 2.96):
> 
> int foo1(void) { return sizeof(struct ext2_sb_info); }
> int foo2(struct ext2_sb_info *sbi) { return sizeof(*sbi); }

The current code is using sizeof(ext2_super_block) for
something which is of type ext2_sb_info.

> yields:
> 
> 00000b70 <foo1>:
>       b70:       b8 dc 00 00 00          mov    $0xdc,%eax
>       b75:       c3                      ret
> 
> 00000b80 <foo2>:
>       b80:       b8 dc 00 00 00          mov    $0xdc,%eax
>       b85:       c3                      ret
> 
> The sizes are the same, so unless it makes a difference with another
> version of gcc then this is just a cosmetic change.

The stylistic change tends to provide insulation from the
above bug.

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