On Fri, 24 May 2019 13:11:44 +0200
Peter Zijlstra <[email protected]> wrote:
> On Mon, May 20, 2019 at 10:20:02AM -0400, Steven Rostedt wrote:
>
> > +#define FGRAPH_RET_SIZE (sizeof(struct ftrace_ret_stack))
> > +#define FGRAPH_RET_INDEX (ALIGN(FGRAPH_RET_SIZE, sizeof(long)) /
> > sizeof(long))
>
> I think you want to write that like:
>
> BUILD_BUG_ON(sizeof(ftrace_ret_stack) % sizeof(long));
Sure.
>
> It'd be very weird for that sizeof not to be right.
Agreed, but I was paranoid. The BUILD_BUG_ON() would also work.
>
> > +#define SHADOW_STACK_SIZE (PAGE_SIZE)
>
> Do we really need that big a shadow stack?
Well, this is a sticky point. I allow up to 16 users at a time
(although I can't imagine more than 5, but you never know), and each
user adds a long and up to 4 more words (which is probably unlikely
anyway). And then we can have deep call stacks (we are getting deeper
each release it seems).
I figured, I start with a page size, and then in the future we can make
it dynamic, or shrink it if it proves to be too much.
>
> > +#define SHADOW_STACK_INDEX \
> > + (ALIGN(SHADOW_STACK_SIZE, sizeof(long)) / sizeof(long))
> > +/* Leave on a buffer at the end */
> > +#define SHADOW_STACK_MAX_INDEX (SHADOW_STACK_INDEX - FGRAPH_RET_INDEX)
> > +
> > +#define RET_STACK(t, index) ((struct ftrace_ret_stack
> > *)(&(t)->ret_stack[index]))
> > +#define RET_STACK_INC(c) ({ c += FGRAPH_RET_INDEX; })
> > +#define RET_STACK_DEC(c) ({ c -= FGRAPH_RET_INDEX; })
>
> I'm thinking something like:
>
> #define RET_PUSH(s, val) \
> do { \
> (s) -= sizeof(val); \
> (typeof(val) *)(s) = val; \
> } while (0)
>
> #define RET_POP(s, type) \
> ({ \
> type *__ptr = (void *)(s); \
> (s) += sizeof(type); \
> *__ptr; \
> })
>
> Would me clearer?
Due to races with interrupts, and this not being an atomic operation, I
had to play tricks with moving the stack pointer and adding data to it.
So I wanted to keep the changing of the stack pointer and adding and
retrieving of the stack data separate.
Later patches remove the RET_STACK_INC/DEC() anyway.
Thanks for taking the time to look at these patches!
-- Steve
