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From:   Alexandre Ghiti <alexghiti@rivosinc.com>
Date:   Thu, 2 Mar 2023 08:57:08 +0100
Message-ID: <CAHVXubgdr_NrLPnZ6NhuDt8uZYZD+R_swpCVGoLDxnD+eOJcuA@mail.gmail.com>
Subject: Re: [PATCH v3 00/24] Remove COMMAND_LINE_SIZE from uapi
To:     Palmer Dabbelt <palmer@dabbelt.com>
Cc:     hca@linux.ibm.com, geert@linux-m68k.org, corbet@lwn.net,
        Richard Henderson <richard.henderson@linaro.org>,
        ink@jurassic.park.msu.ru, mattst88@gmail.com, vgupta@kernel.org,
        linux@armlinux.org.uk, Catalin Marinas <catalin.marinas@arm.com>,
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        kernel@xen0n.name, monstr@monstr.eu, tsbogend@alpha.franken.de,
        James.Bottomley@hansenpartnership.com, deller@gmx.de,
        mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu,
        Paul Walmsley <paul.walmsley@sifive.com>,
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On Thu, Mar 2, 2023 at 4:17=E2=80=AFAM Palmer Dabbelt <palmer@dabbelt.com> =
wrote:
>
> On Tue, 14 Feb 2023 01:19:02 PST (-0800), hca@linux.ibm.com wrote:
> > On Tue, Feb 14, 2023 at 09:58:17AM +0100, Geert Uytterhoeven wrote:
> >> Hi Heiko,
> >>
> >> On Tue, Feb 14, 2023 at 9:39 AM Heiko Carstens <hca@linux.ibm.com> wro=
te:
> >> > On Tue, Feb 14, 2023 at 08:49:01AM +0100, Alexandre Ghiti wrote:
> >> > > This all came up in the context of increasing COMMAND_LINE_SIZE in=
 the
> >> > > RISC-V port.  In theory that's a UABI break, as COMMAND_LINE_SIZE =
is the
> >> > > maximum length of /proc/cmdline and userspace could staticly rely =
on
> >> > > that to be correct.
> >> > >
> >> > > Usually I wouldn't mess around with changing this sort of thing, b=
ut
> >> > > PowerPC increased it with a5980d064fe2 ("powerpc: Bump COMMAND_LIN=
E_SIZE
> >> > > to 2048").  There are also a handful of examples of COMMAND_LINE_S=
IZE
> >> > > increasing, but they're from before the UAPI split so I'm not quit=
e sure
> >> > > what that means: e5a6a1c90948 ("powerpc: derive COMMAND_LINE_SIZE =
from
> >> > > asm-generic"), 684d2fd48e71 ("[S390] kernel: Append scpdata to ker=
nel
> >> > > boot command line"), 22242681cff5 ("MIPS: Extend COMMAND_LINE_SIZE=
"),
> >> > > and 2b74b85693c7 ("sh: Derive COMMAND_LINE_SIZE from
> >> > > asm-generic/setup.h.").
> >> > >
> >> > > It seems to me like COMMAND_LINE_SIZE really just shouldn't have b=
een
> >> > > part of the uapi to begin with, and userspace should be able to ha=
ndle
> >> > > /proc/cmdline of whatever length it turns out to be.  I don't see =
any
> >> > > references to COMMAND_LINE_SIZE anywhere but Linux via a quick Goo=
gle
> >> > > search, but that's not really enough to consider it unused on my e=
nd.
> >> > >
> >> > > The feedback on the v1 seemed to indicate that COMMAND_LINE_SIZE r=
eally
> >> > > shouldn't be part of uapi, so this now touches all the ports.  I'v=
e
> >> > > tried to split this all out and leave it bisectable, but I haven't
> >> > > tested it all that aggressively.
> >> >
> >> > Just to confirm this assumption a bit more: that's actually the same
> >> > conclusion that we ended up with when commit 3da0243f906a ("s390: ma=
ke
> >> > command line configurable") went upstream.
>
> Thanks, I guess I'd missed that one.  At some point I think there was
> some discussion of making this a Kconfig for everyone, which seems
> reasonable to me -- our use case for this being extended is syzkaller,
> but we're sort of just picking a value that's big enough for now and
> running with it.
>
> Probably best to get it out of uapi first, though, as that way at least
> it's clear that it's not uABI.
>
> >> Commit 622021cd6c560ce7 ("s390: make command line configurable"),
> >> I assume?
> >
> > Yes, sorry for that. I got distracted while writing and used the wrong
> > branch to look this up.
>
> Alex: Probably worth adding that to the list in the cover letter as it
> looks like you were planning on a v4 anyway (which I guess you now have
> to do, given that I just added the issue to RISC-V).

Yep, I will :)

Thanks,

Alex