Re: [PATCH] printk: Userspace format enumeration support

Chris Down
2021-02-06T02:33:15Z

Hi Steven,

Steven Rostedt writes:
>Interesting, because when I was looking at the original patch (looked at
>the lore link before reading your reply), I thought to myself "this looks
>exactly like what I did for trace_printk formats", which the above file is
>where it is shown. I'm curious if this work was inspired by that?

The double __builtin_constant_p() trick was suggested by Johannes based on
prior art in trace_puts() just prior to patch submission. Other than that, it
seems we came up with basically the same solution independently. :-)

>> Anyway, there is something wrong at the moment. The output looks fine
>> with cat. But "less" says that it is a binary format and the output
>> is a bit messy:
>
>Hmm, that's usually the case when lseek gets messed up. Not sure how that
>happened.

It looks as intended to me -- none of the newlines, nulls, or other control
sequences are escaped currently, since I didn't immediately see a reason to do
that. If that's a blocker though, I'm happy to change it.

>> $> less /proc/printk_formats
>> "/proc/printk_formats" may be a binary file. See it anyway?
>> vmlinux,^A3Warning: unable to open an initial console.
>> ^@vmlinux,^A3Failed to execute %s (error %d)
>> ^@vmlinux,^A6Kernel memory protection disabled.
>> ^@vmlinux,^A3Starting init: %s exists but couldn't execute it (error %d)
>>
>>
>> That is for now. I still have to think about it. And I am also curious
>> about what others thing about this idea.
>>
>
>I'm not against the idea. I don't think it belongs in /proc. Perhaps
>debugfs is a better place to put it.

Any location is fine with me, as long as it gets to userspace. How does
<debugfs>/printk/formats or <debugfs>/printk/formats/<module> sound to you?

Thanks,

Chris