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

["H. Peter Anvin" <hpa@zytor.com>]

On March 1, 2023 7:17:18 PM PST, 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> 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).

The only use that is uapi is the *default* length of the command line if the kernel header doesn't include it (in the case of x86, it is in the bzImage header, but that is atchitecture- or even boot format-specific.)