[ Trimmed the cc list because my SMTP didn't accept that many
addresses. ]

On Sun, 26 Aug 2018 13:25:14 -0700
Linus Torvalds [off-list ref] wrote:

On Sun, Aug 26, 2018 at 12:32 PM H. Peter Anvin [off-list ref] wrote:

quoted

Here is a full-blown (user space) test program demonstrating the whole
technique and how to use it.  

So while I agree that some _THIS_IP_ users might be better off being
converted to __builtin_return_address(0) at the caller, I also think
that the whole "notailcall" thing shows why that can easily be more
problematic than just our currnet _THIS_IP_ solution.

Honestly, I'd suggest:

 - just do the current_text_addr() to _THIS_IP_ conversion

 - keep _THIS_IP_ and make it be the generic one, and screw the whole
"some architectures might implement is better" issue. Nobody cares.

 - try to convince people to move away from the "we want the kernel
instruction pointer for the call" model entirely, and consider this a
"legacy" issue.

The whole instruction pointer is a nasty thing. We should discourage
it and not make complex infrastructure for it.

Instead, maybe we could encourage something like

  struct kernel_loc { const char *file; const char *fn; int line; };

  #define __GEN_LOC__(n) \
        ({ static const struct kernel_loc n = { \
                __FILE__, __FUNCTION__, __LINE__  \
           }; &n; })

  #define _THIS_LOC_ __GEN_LOC__(__UNIQUE_ID(loc))

which is a hell of a lot nicer to use, and actually allows gcc to
optimize things (try it: if you pass a _THIS_LOC_ off to an inline
function, and that inline function uses the name and line number, gcc
will pick them up directly, without the extra structure dereference.

Wouldn't it be much nicer to pass these kinds of "location pointer"
around, rather than the nasty _THIS_IP_ thing?

Seems nice. Do you even need this unique ID thing? AFAIKS the name
would never really be useful.

It could perhaps go into a cold data section too, I assume the common
case is that you do not access it. Although gcc will end up putting
the file and function names into regular rodata.

Possibly we could add a printk specifier for it, pass it through to
existing BUG, etc macros that want exactly this, etc. Makes a lot of
sense.

Thanks,
Nick