Re: Dangerous use of alloca

[Agustina Arzille]

Hello, Samuel.

On 2016-03-15 13:31, Samuel Thibault wrote:
> Justus Winter, on Tue 15 Mar 2016 14:39:00 +0100, wrote:
> > Quoting Agustina Arzille (2016-03-15 14:11:27)
> > > Hello, everyone.
> > >
> > > I was browsing some gnumach source files, and noticed some dangerous use
> > > of the gcc builtin 'alloca'. In the file kern/bootstrap.c, lines 212 and
> > > 228, we have the following statement:
> > >
> > > memcpy (alloca (len), ...)
> >
> > Good catch.  However, alloca is dangerous even if used correctly, and
> > is best be avoided.
>
> What do you mean by "dangerous"?  I don't think we'll ever get a 
> command
> line that does not fit into the stack.
>
> Samuel

That's not what I meant. Stack overflow is clearly very unlikely, if not
impossible. What I meant about 'dangerous' is the fact that alloca is 
being
used inside the list of arguments of a function call, which can lead to
subtle bugs, because alloca modifies the SP register, which is also 
being
modified by the function arguments being pushed into the stack.

See: http://man7.org/linux/man-pages/man3/alloca.3.html ,
specifically, the section 'BUGS'.