Re: [PATCH v7 19/36] function_graph: Implement fgraph_reserve_data() and fgraph_retrieve_data()

Masami Hiramatsu
2024-02-14T23:45:58Z

On Wed, 14 Feb 2024 13:59:58 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 7 Feb 2024 00:11:01 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
>
> > From: Ste
> > +/**
> > + * fgraph_reserve_data - Reserve storage on the task's ret_stack
> > + * @idx: The index of fgraph_array
> > + * @size_bytes: The size in bytes to reserve
> > + *
> > + * Reserves space of up to FGRAPH_MAX_DATA_SIZE bytes on the
> > + * task's ret_stack shadow stack, for a given fgraph_ops during
> > + * the entryfunc() call. If entryfunc() returns zero, the storage
> > + * is discarded. An entryfunc() can only call this once per iteration.
> > + * The fgraph_ops retfunc() can retrieve this stored data with
> > + * fgraph_retrieve_data().
> > + *
> > + * Returns: On success, a pointer to the data on the stack.
> > + * Otherwise, NULL if there's not enough space left on the
> > + * ret_stack for the data, or if fgraph_reserve_data() was called
> > + * more than once for a single entryfunc() call.
> > + */
> > +void *fgraph_reserve_data(int idx, int size_bytes)
> > +{
> > + unsigned long val;
> > + void *data;
> > + int curr_ret_stack = current->curr_ret_stack;
> > + int data_size;
> > +
> > + if (size_bytes > FGRAPH_MAX_DATA_SIZE)
> > + return NULL;
> > +
> > + /* Convert to number of longs + data word */
> > + data_size = DIV_ROUND_UP(size_bytes, sizeof(long));
>
> Hmm, the above is a fast path. I wonder if we should add a patch to make that into:
>
> if (unlikely(size_bytes & (sizeof(long) - 1)))
> data_size = DIV_ROUND_UP(size_bytes, sizeof(long));
> else
> data_size = size_bytes >> (sizeof(long) == 4 ? 2 : 3);
>
> to keep from doing the division.

OK, I thought DIV_ROUND_UP was not much cost. Since sizeof(long) is
fixed 4 or 8, so

data_size = (size_bytes + sizeof(long) - 1) >> BITS_PER_LONG;

will this work?

Thanks,

>
> -- Steve
>
> > +
> > + val = get_fgraph_entry(current, curr_ret_stack - 1);
> > + data = &current->ret_stack[curr_ret_stack];
> > +
> > + curr_ret_stack += data_size + 1;
> > + if (unlikely(curr_ret_stack >= SHADOW_STACK_MAX_INDEX))
> > + return NULL;
> > +
> > + val = make_fgraph_data(idx, data_size, __get_index(val) + data_size + 1);
> > +
> > + /* Set the last word to be reserved */
> > + current->ret_stack[curr_ret_stack - 1] = val;
> > +
> > + /* Make sure interrupts see this */
> > + barrier();
> > + current->curr_ret_stack = curr_ret_stack;
> > + /* Again sync with interrupts, and reset reserve */
> > + current->ret_stack[curr_ret_stack - 1] = val;
> > +
> > + return data;
> > +}
> > +

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>