Re: [PATCH] riscv: Bump COMMAND_LINE_SIZE value to 1024

[Palmer Dabbelt]

On Fri, 02 Apr 2021 11:33:30 PDT (-0700), macro@xxxxxxxxxxx wrote:
> On Fri, 2 Apr 2021, David Abdurachmanov wrote:
>
> > > > >  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.
>
>  The macro is in an include/uapi header, so it's exported to the userland
> and a part of the user API.  I don't know what the consequences are for
> the RISC-V port specifically, but it has raised my attention, and I think
> it has to be investigated.
>
>  Perhaps it's OK to change it after all, but you'd have to go through
> known/potential users of this macro.  I guess there shouldn't be that many
> of them.
>
>  In any case it cannot depend on Kconfig, because the userland won't have
> access to the configuration, and then presumably wants to handle any and
> all.

It kind of feels to me like COMMAND_LINE_SIZE shouldn't have been part 
of the UABI to begin with.  I sent a patch to remove it from the 
asm-generic UABI, let's see if anyone knows of a reason it should be 
UABI:

https://lore.kernel.org/linux-arch/20210423025545.313965-1-palmer@xxxxxxxxxxx/T/#u