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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Fri, 2 Apr 2021 10:40:58 +0200
Message-ID: <CACT4Y+Z0PaAuUFrOBenztWkw8OV=J-qaeD1FASPM4ufcLg5a5A@mail.gmail.com>
Subject: Re: [PATCH] riscv: Bump COMMAND_LINE_SIZE value to 1024
To:     Palmer Dabbelt <palmer@dabbelt.com>
Cc:     macro@orcam.me.uk, Alex Ghiti <alex@ghiti.fr>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Apr 2, 2021 at 6:37 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Tue, 30 Mar 2021 13:31:45 PDT (-0700), macro@orcam.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?