Re: [PATCH v3 00/24] Remove COMMAND_LINE_SIZE from uapi

Palmer Dabbelt
2023-03-02T03:17:31Z

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> wrote:
>> > 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, but
>> > > PowerPC increased it with a5980d064fe2 ("powerpc: Bump COMMAND_LINE_SIZE
>> > > to 2048"). There are also a handful of examples of COMMAND_LINE_SIZE
>> > > increasing, but they're from before the UAPI split so I'm not quite sure
>> > > what that means: e5a6a1c90948 ("powerpc: derive COMMAND_LINE_SIZE from
>> > > asm-generic"), 684d2fd48e71 ("[S390] kernel: Append scpdata to kernel
>> > > 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 been
>> > > part of the uapi to begin with, and userspace should be able to handle
>> > > /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 Google
>> > > search, but that's not really enough to consider it unused on my end.
>> > >
>> > > The feedback on the v1 seemed to indicate that COMMAND_LINE_SIZE really
>> > > shouldn't be part of uapi, so this now touches all the ports. I've
>> > > 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: make
>> > 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).