Re: [PATCH v7 03/10] drm/xe/devcoredump: Add ASCII85 dump helper function

[John Harrison <John@Hendrik.org.uk>]

On 9/5/2024 20:04, Lucas De Marchi wrote:
> On Thu, Sep 05, 2024 at 07:01:33PM GMT, John Harrison wrote:
>> On 9/5/2024 18:54, Lucas De Marchi wrote:
>>> On Thu, Sep 05, 2024 at 01:50:58PM GMT, John.C.Harrison@Intel.com 
>>> wrote:
>>>> From: John Harrison <John.C.Harrison@Intel.com>
>>>>
>>>> There is a need to include the GuC log and other large binary objects
>>>> in core dumps and via dmesg. So add a helper for dumping to a printer
>>>> function via conversion to ASCII85 encoding.
>>>
>>> why are we not dumping the binary data directly to devcoredump?
>> As per earlier comments, there is a WiFi driver or some such that 
>> does exactly that. But all they are dumping is a binary blob.
>
> In your v5 I see you mentioned
> drivers/net/wireless/ath/ath10k/coredump.c, but that is a precedence for
> including it as is from the device rather converting it to ASCII85 or
> something else. It seems odd to do that type of conversion in kernel
> space when it could be perfectly done in userspace.
It really can't. An end user could maybe be expected to zip or tar a 
coredump file before attaching it to a bug report but they are certainly 
not going to try to ASCII85 encode random bits of it. Whereas, putting 
that in the kernel means it is just there. It is done. And it is pretty 
trivial - just call a helper function and it does everything for you. 
Also, I very much doubt you can spew raw binary data via dmesg. Even if 
the kernel would print it for you (which I doubt), the user tools like 
syslogd and dmesg itself are going to filter it to make it ASCII safe.

The i915 error dumps have been ASCII85 encoded using the kernel's 
ASCII85 encoding helper function since forever. This patch is just a 
wrapper around the kernel's existing implementation in order to make it 
more compatible with printing to dmesg. This is not creating a new 
precedent. It already exists.

>
> $ git grep ascii85.h
> drivers/gpu/drm/i915/i915_gpu_error.c:#include <linux/ascii85.h>
> drivers/gpu/drm/msm/adreno/a6xx_gpu_state.c:#include <linux/ascii85.h>
> drivers/gpu/drm/msm/adreno/adreno_gpu.c:#include <linux/ascii85.h>
> drivers/gpu/drm/xe/xe_lrc.c:#include <linux/ascii85.h>
> drivers/gpu/drm/xe/xe_vm.c:#include <linux/ascii85.h>
And the list of drivers which dump raw binary data in a coredump file 
is... ath10k. ASCII85 wins 3 to 1.


>
>>
>> We want the devcoredump file to still be human readable. That won't 
>> be the case if you stuff binary data in the middle of it. Most 
>> obvious problem - the zeros in the data will terminate your text file 
>> at that point. Potentially bigger problem for end users - random fake 
>> ANSI codes will destroy your terminal window if you try to cat the 
>> file to read it.
>
> Users don't get a coredump and cat it to the terminal.
> =(lk%A8`T7AKYH#FD,6++EqOABHUhsG%5H2ARoq#E$/V$Bl7Q+@<5pmBe<q;Bk;0mCj@.3DIal2FD5Q-+E_RBART+X@VfTuGA2/4Dfp.E@3BN0DfB9.+E1b0F(KAV+:8 
>
>
> Lucas De Marchi
They might. Either intentionally or accidentally. I've certainly done it 
myself. And people will certainly want to look at it in any random 
choice of text editor, pager, etc. 'cos you know, it is meant to be read 
by humans. If it is full of binary data then that becomes even more 
difficult than simply being full of ASCII gibberish. No matter what you 
are doing, the ASCII version is safer and easier to look at the rest of 
the file around it.

I don't understand why you are so desperate to have raw binary data in 
the middle of a text file. The disadvantages are multiple but the only 
advantage is a slightly smaller file. And the true route to smaller 
files is to add compression like we have in i915.

John.