Re: [PATCH] vsprintf: Make %pGp print the hex value

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On 12/10/2021 13.32, Matthew Wilcox wrote:
> On Tue, Oct 12, 2021 at 11:55:50AM +0200, Rasmus Villemoes wrote:
>> ...default_flag_spec has '.flags = SPECIAL | SMALL,', i.e. what one
>> would get from %#x . I'm guessing that's what upsets 0-day.
>>
>> Geez it would be nice if 0day actually reported the "Expected foo, but
>> bar was emitted to the buffer".
> 
> It's there; you just have to dive into the dmesg.xz to get it:
> 
> [   48.607787][ T1214] test_printf: vsnprintf(buf, 256, "%pGp", ...) returned 33, expected 29
> [   48.616106][ T1214] test_printf: vsnprintf(buf, 18, "%pGp", ...) returned 33, expected 29
> [   48.624292][ T1214] test_printf: vsnprintf(buf, 0, "%pGp", ...) returned 33, expected 29
> [   48.632403][ T1214] test_printf: kvasprintf(..., "%pGp", ...) returned '0x0(node=0|zone=0|lastcpupid=0x0)', expected 'node=0|zone=0|lastcpupid=0x0)'
> [   48.645741][ T1214] test_printf: vsnprintf(buf, 256, "%pGp", ...) returned 33, expected 29
> [   48.654023][ T1214] test_printf: vsnprintf(buf, 23, "%pGp", ...) returned 33, expected 29
> [   48.662218][ T1214] test_printf: vsnprintf(buf, 0, "%pGp", ...) returned 33, expected 29
> [   48.670327][ T1214] test_printf: kvasprintf(..., "%pGp", ...) returned '0x0(node=0|zone=0|lastcpupid=0x0)', expected 'node=0|zone=0|lastcpupid=0x0)'
> [   48.683670][ T1214] test_printf: vsnprintf(buf, 256, "%pGp", ...) returned 88, expected 71
> [   48.691937][ T1214] test_printf: vsnprintf(buf, 64, "%pGp", ...) returned 88, expected 71
> [   48.700128][ T1214] test_printf: vsnprintf(buf, 0, "%pGp", ...) returned 88, expected 71
> [   48.708249][ T1214] test_printf: kvasprintf(..., "%pGp", ...) returned '0x4fffffc008003c(uptodate|dirty|lru|active|swapbacked|node=1|zone=1|lastcpupid=0x1fffff)', expected 'uptodate|dirty|lru|active|swapbacked|node=1|zone=1|lastcpupid=0x1fffff)'
> [   48.730275][ T1214] test_printf: failed 12 out of 420 tests
> 
> So you're right, it's missing the 0x, 

Ah, it's actually missing the whole '0xfoo(' because you're not updating
'size' after the first

+	snprintf(cmp_buf + size, BUF_SIZE - size, "%x(", flags);

so the next write to cmp_buf overwrites whatever you wrote here.

> but I still don't know how to run this self-test.

Neither do I, when I created test_printf.c I made sure it ran on boot if
built-in, and that's certainly how I've been testing vsprintf.c patches
in the past. Maybe the kstm crowd can explain how to get that to work as
expected.

Rasmus