Re: modutils-2.3.15 'insmod'

[Kurt Wall <kwall@kurtwerks.com>]

Quoth Richard B. Johnson:
> 
> modutils-2.3.15, and probably later, has a bug that can prevent
> modules from being loaded from initrd, this results in not
> being able to mount a root file-system. The bug assumes that
> malloc() will return a valid pointer when given an allocation
> size of zero.

This isn't a bug. The standard allow returning a non-null pointer
for malloc(0).

> When there are no modules loaded, insmod scans for modules
> and allocates data using its xmalloc() based upon the number
> of modules found. If the number was 0, it attempts to allocate
> 0 bytes (0 times the size of a structure). If malloc() returns
> NULL (and it can, probably should), xmalloc() will write an
> "out of memory" diagnostic and call exit().
> 
> The most recent `man` pages that RH 9.0 distributes states that
> malloc() can return either NULL of a pointer that is valid for
> free(). This, of course, depends upon the 'C' runtime library's
> malloc() implementation.

Perhaps, but IIRC, the rationale in the GNU C library was that
existing programs assume malloc(0) != 0, which allows you to call
realloc on the pointer. Returning NULL only makes sense if the 
malloc() call fails.

> It is likely that malloc(0) returning a valid pointer is a bug
> that has prevented this problem from being observed. Such a
> bug in malloc() is probably necessary to keep legacy software

Not a bug. Bad design perhaps, but not a bug.

> running, but new software shouldn't use such atrocious side-effects.
> An allocation of zero needs to be discovered and fixed early
> in code design.

Kurt
-- 
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