linux-kernel - Re: [PATCH] riscv: Bump COMMAND_LINE

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Date:   Fri, 2 Apr 2021 11:58:40 +0300
From:   David Abdurachmanov <david.abdurachmanov@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>, macro@...am.me.uk,
        Alex Ghiti <alex@...ti.fr>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv: Bump COMMAND_LINE_SIZE value to 1024

On Fri, Apr 2, 2021 at 11:43 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Fri, Apr 2, 2021 at 6:37 AM Palmer Dabbelt <palmer@...belt.com> wrote:
> >
> > On Tue, 30 Mar 2021 13:31:45 PDT (-0700), macro@...am.me.uk wrote:
> > > On Mon, 29 Mar 2021, Palmer Dabbelt wrote:
> > >
> > >> > --- /dev/null
> > >> > +++ b/arch/riscv/include/uapi/asm/setup.h
> > >> > @@ -0,0 +1,8 @@
> > >> > +/* SPDX-License-Identifier: GPL-2.0-only WITH Linux-syscall-note */
> > >> > +
> > >> > +#ifndef _UAPI_ASM_RISCV_SETUP_H
> > >> > +#define _UAPI_ASM_RISCV_SETUP_H
> > >> > +
> > >> > +#define COMMAND_LINE_SIZE 1024
> > >> > +
> > >> > +#endif /* _UAPI_ASM_RISCV_SETUP_H */
> > >>
> > >> I put this on fixes, but it seemes like this should really be a Kconfig
> > >> enttry.  Either way, ours was quite a bit smaller than most architectures and
> > >> it's great that syzbot has started to find bugs, so I'd rather get this in
> > >> sooner.
> > >
> > >  This macro is exported as a part of the user API so it must not depend on
> > > Kconfig.  Also changing it (rather than say adding COMMAND_LINE_SIZE_V2 or
> > > switching to an entirely new data object that has its dimension set in a
> > > different way) requires careful evaluation as external binaries have and
> > > will have the value it expands to compiled in, so it's a part of the ABI
> > > too.
> >
> > Thanks, I didn't realize this was part of the user BI.  In that case we
> > really can't chage it, so we'll have to sort out some other way do fix
> > whatever is going on.
> >
> > I've dropped this from fixes.
>
> Does increasing COMMAND_LINE_SIZE break user-space binaries? I would
> expect it to work the same way as adding new enum values, or adding
> fields at the end of versioned structs, etc.
> I would assume the old bootloaders/etc will only support up to the
> old, smaller max command line size, while the kernel will support
> larger command line size, which is fine.
> However, if something copies /proc/cmdline into a fixed-size buffer
> and expects that to work, that will break... that's quite unfortunate
> user-space code... is it what we afraid of?
>
> Alternatively, could expose the same COMMAND_LINE_SIZE, but internally
> support a larger command line?

Looking at kernel commit history I see PowerPC switched from 512 to
2048, and I don't see complaints about the ABI on the mailing list.

If COMMAND_LINE_SIZE is used by user space applications and we
increase it there shouldn't be problems. I would expect things to
work, but just get truncated boot args? That is the application will
continue only to look at the initial 512 chars.

https://linuxppc-dev.ozlabs.narkive.com/m4cj8nBa/patch-1-1-powerpc-increase-command-line-size-to-2048-from-512