On Wed, 26 Nov 2008 11:46:58 -0500 (EST) Steven Rostedt <[EMAIL PROTECTED]>
wrote:
>
>
> On Tue, 25 Nov 2008, Andrew Morton wrote:
>
> > On Wed, 26 Nov 2008 00:16:24 -0500 Steven Rostedt <[EMAIL PROTECTED]> wrote:
> >
> > > From: Steven Rostedt <[EMAIL PROTECTED]>
> > >
> > > Impact: more efficient code for ftrace graph tracer
> > >
> > > This patch uses the dynamic patching, when available, to patch
> > > the function graph code into the kernel.
> > >
> > > This patch will ease the way for letting both function tracing
> > > and function graph tracing run together.
> > >
> > > ...
> > >
> > > +static int ftrace_mod_jmp(unsigned long ip,
> > > + int old_offset, int new_offset)
> > > +{
> > > + unsigned char code[MCOUNT_INSN_SIZE];
> > > +
> > > + if (probe_kernel_read(code, (void *)ip, MCOUNT_INSN_SIZE))
> > > + return -EFAULT;
> > > +
> > > + if (code[0] != 0xe9 || old_offset != *(int *)(&code[1]))
> >
> > erk. I suspect that there's a nicer way of doing this amongst our
> > forest of get_unaligned_foo() interfaces. Harvey will know.
>
> Hmm, I may be able to make a struct out of code.
>
> struct {
> unsigned char op;
> unsigned int offset;
> } code __attribute__((packed));
>
> Would that look better?
nah, let's do something more generic for this.
> >
> > > + return -EINVAL;
> > > +
> > > + *(int *)(&code[1]) = new_offset;
> >
> > Might be able to use put_unaligned_foo() here.
> >
> > The problem is that these functions use sizeof(*ptr) to work out what
> > to do, so a cast is still needed. A get_unaligned32(ptr) would be
> > nice. One which takes a void* and assumes CPU ordering.
>
> Is there a correctness concern here? This is arch specific code, so I'm
> not worried about other archs.
No, the code is OK as-is.
It's just that "read a word from an [maybe-]unaligned address" is such
a common operation that there should be a nice clean simple function to
do it, rather than doing open-coded (and different) weird C tricks at each
and\ every site.
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