linux-kernel - Re: [PATCH v2] kbuild: treat char as always unsigned

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Message-ID: <Y1asm2GnqgXirEIy@zx2c4.com>
Date:   Mon, 24 Oct 2022 17:17:47 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-toolchains@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] kbuild: treat char as always unsigned

On Mon, Oct 24, 2022 at 12:24:24PM +0300, Dan Carpenter wrote:
> On Wed, Oct 19, 2022 at 02:30:34PM -0600, Jason A. Donenfeld wrote:
> > Recently, some compile-time checking I added to the clamp_t family of
> > functions triggered a build error when a poorly written driver was
> > compiled on ARM, because the driver assumed that the naked `char` type
> > is signed, but ARM treats it as unsigned, and the C standard says it's
> > architecture-dependent.
> > 
> > I doubt this particular driver is the only instance in which
> > unsuspecting authors make assumptions about `char` with no `signed` or
> > `unsigned` specifier. We were lucky enough this time that that driver
> > used `clamp_t(char, negative_value, positive_value)`, so the new
> > checking code found it, and I've sent a patch to fix it, but there are
> > likely other places lurking that won't be so easily unearthed.
> > 
> > So let's just eliminate this particular variety of heisensign bugs
> > entirely. Set `-funsigned-char` globally, so that gcc makes the type
> > unsigned on all architectures.
> > 
> > This will break things in some places and fix things in others, so this
> > will likely cause a bit of churn while reconciling the type misuse.
> > 
> 
> This is a very daring change and obviously is going to introduce bugs.
> It might be better to create a static checker rule that says "char"
> without explicit signedness can only be used for strings.

Indeed this would be great.

> 
> arch/parisc/kernel/drivers.c:337 print_hwpath() warn: impossible condition '(path->bc[i] == -1) => (0-255 == (-1))'
> arch/parisc/kernel/drivers.c:410 setup_bus_id() warn: impossible condition '(path.bc[i] == -1) => (0-255 == (-1))'
> arch/parisc/kernel/drivers.c:486 create_parisc_device() warn: impossible condition '(modpath->bc[i] == -1) => (0-255 == (-1))'
> arch/parisc/kernel/drivers.c:759 hwpath_to_device() warn: impossible condition '(modpath->bc[i] == -1) => (0-255 == (-1))'
> drivers/media/dvb-frontends/stv0288.c:471 stv0288_set_frontend() warn: assigning (-9) to unsigned variable 'tm'
> drivers/media/dvb-frontends/stv0288.c:471 stv0288_set_frontend() warn: we never enter this loop
> drivers/misc/sgi-gru/grumain.c:711 gru_check_chiplet_assignment() warn: 'gts->ts_user_chiplet_id' is unsigned
> drivers/net/wireless/cisco/airo.c:5316 proc_wepkey_on_close() warn: assigning (-16) to unsigned variable 'key[i / 3]'
> drivers/net/wireless/ralink/rt2x00/rt2800lib.c:9415 rt2800_iq_search() warn: assigning (-32) to unsigned variable 'idx0'
> drivers/net/wireless/ralink/rt2x00/rt2800lib.c:9470 rt2800_iq_search() warn: assigning (-32) to unsigned variable 'perr'
> drivers/video/fbdev/sis/init301.c:3549 SiS_GetCRT2Data301() warn: 'SiS_Pr->SiS_EModeIDTable[ModeIdIndex]->ROMMODEIDX661' is unsigned
> sound/pci/au88x0/au88x0_core.c:2029 vortex_adb_checkinout() warn: signedness bug returning '(-22)'
> sound/pci/au88x0/au88x0_core.c:2046 vortex_adb_checkinout() warn: signedness bug returning '(-12)'
> sound/pci/au88x0/au88x0_core.c:2125 vortex_adb_allocroute() warn: 'vortex_adb_checkinout(vortex, (0), en, 0)' is unsigned
> sound/pci/au88x0/au88x0_core.c:2170 vortex_adb_allocroute() warn: 'vortex_adb_checkinout(vortex, stream->resources, en, 4)' is unsigned
> sound/pci/rme9652/hdsp.c:3953 hdsp_channel_buffer_location() warn: 'hdsp->channel_map[channel]' is unsigned
> sound/pci/rme9652/rme9652.c:1833 rme9652_channel_buffer_location() warn: 'rme9652->channel_map[channel]' is unsigned


Thanks. I'll fix these up.

Jason