linux-kernel - Re: [PATCH v2 2/4] usb: typec: tps6598x: Add trace event for status register

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Message-ID: <20210214204127.ezrlr4u76onqcxl7@mail>
Date:   Sun, 14 Feb 2021 21:41:27 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ramsay Jones <ramsay@...sayjones.plus.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-usb@...r.kernel.org,
        Guido Günther <agx@...xcpu.org>,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] usb: typec: tps6598x: Add trace event for status
 register

On Sun, Feb 14, 2021 at 11:00:48AM -0800, Linus Torvalds wrote:
> On Sun, Feb 14, 2021 at 10:42 AM Ramsay Jones
> <ramsay@...sayjones.plus.com> wrote:
> >
> > >
> > > I looked around but didn't find any hints how to fix this. Any pointers
> > > I missed (added the sparse list to cc:)?
> >
> > This is a limitation of sparse; when using the 'stringize' pre-processor
> > operator #, the maximum size of the resulting string is about 8k (if I
> > remember correctly).
> 
> Well, yes and no.
> 
> The C89 standard actually says that a string literal can be at most
> 509 characters to be portable. C99 increased it to 4095 characters.
> 
> Sparse makes the limit higher, and the limit could easily be expanded
> way past 8kB - but the point is that large string literals are
> actually not guaranteed to be valid C.
> 
> So honestly, it really sounds like that TRACE_EVENT() thing is doing
> something it shouldn't be doing.

In itself, it's OKish but it does a lot of macro expansions and most
arguments are macros of macros of ... but the problem seems to be
limited to TP_printk().

In the current case, the offender is the string 'print_fmt_tps6598x_status'
which is just under 26K long especially because it expand
TPS6598X_STATUS_FLAGS_MASK but also because the arguments use FIELD_GET()
and thus __BF_FIELD_CHECK().
> 
> I don't think there's any fundamental limit why sparse does 8kB as a
> limit (just a few random buffers). Making sparse accept larger ones
> should be as simple as just increasing MAX_STRING, but I really don't
> think the kernel should encourage that kind of excessive string sizes.

Like you noted, there are just a few cases in the kernel and IIRC
there is or was one case in it too.
I would tend to increase MAX_STRING to something like 32 or 64K,
in order to keep it reasonable but let sparse to continue its processing,
but add a warning when the string/token is bigger than the current 8K.

-- Luc