linux-kernel - Re: [PATCH v10-mte 4/7] arm64: mte: implement CONFIG_ARM64_MTE

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Message-ID: <ZXyAXPxlmq11rp2Y@yury-ThinkPad>
Date: Fri, 15 Dec 2023 08:35:40 -0800
From: Yury Norov <yury.norov@...il.com>
To: Alexander Potapenko <glider@...gle.com>
Cc: catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
	andreyknvl@...il.com, andriy.shevchenko@...ux.intel.com,
	aleksander.lobakin@...el.com, linux@...musvillemoes.dk,
	alexandru.elisei@....com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
	syednwaris@...il.com, william.gray@...aro.org
Subject: Re: [PATCH v10-mte 4/7] arm64: mte: implement CONFIG_ARM64_MTE_COMP

On Fri, Dec 15, 2023 at 04:19:27PM +0100, Alexander Potapenko wrote:
> >
> > That looks weird... You're casting address of a 'data' to a bitmap
> > instead of 'data'. At the 1st glance it makes little sense because
> > 'data' is passed as parameter. Moreover, in mte_is_compressed()
> > you pass 'data', not '&data'. Can you please comment on your
> > intention?
> 
> Although `data` is a void*, it actually contains 64 bits of compressed
> data, so we pass &data to mte_bitmap_read() to read its contents.
> Perhaps I'd better make `data` an unsigned long to avoid confusion.
 
Still don't understand. Let's consider this example:

yury:linux$ cat tst.c
#include <stdio.h>

unsigned long data[1] = {0xabc};

void foo(unsigned long *data)
{
	printf("foo:  *data\t%lx\n", (unsigned long)*data);
	printf("foo:   data\t%lx\n",  (unsigned long)data);
	printf("foo:  &data\t%lx\n", (unsigned long)&data);
}

void bar(unsigned long *data)
{
	volatile unsigned long x[100];

	printf("bar:  *data\t%lx\n", (unsigned long)*data);
	printf("bar:   data\t%lx\n",  (unsigned long)data);
	printf("bar:  &data\t%lx\n", (unsigned long)&data);
}

int main(int argc, char *argv[])
{

	foo(data);
	bar(data);

	printf("main: *data\t%lx\n", (unsigned long)*data);
	printf("main:  data\t%lx\n",  (unsigned long)data);
	printf("main: &data\t%lx\n", (unsigned long)&data);

	return 0;
}
yury:linux$ gcc tst.c -O0
yury:linux$ ./a.out
foo:  *data	abc
foo:   data	555b2cef9010
foo:  &data	7fff39d6e5f8
bar:  *data	abc
bar:   data	555b2cef9010
bar:  &data	7fff39d6e2c8
main: *data	abc
main:  data	555b2cef9010
main: &data	555b2cef9010

Data and *data have their meaning across scope boundary: a pointer and
a content. The &data is pretty much a random number - a pointer to
somewhere on a function's stack. Isn't?

> > > +     max_ranges = MTE_MAX_RANGES;
> > > +     /* Skip the leading bit indicating the inline case. */
> > > +     mte_bitmap_read(bitmap, &bit_pos, 1);
> > > +     largest_idx =
> > > +             mte_bitmap_read(bitmap, &bit_pos, MTE_BITS_PER_LARGEST_IDX);
> >
> > Nit: really no need to split the line - we're OK with 100-chars per
> > line now.
> 
> That's true, but I am relying on clang-format here (maybe we should
> extend the limit in .clang-format?)

If clang-format hurts readability, don't use it.

Thanks,
Yury