On Fri, Apr 2, 2021 at 11:43 AM Dmitry Vyukov [off-list ref] wrote:

On Fri, Apr 2, 2021 at 6:37 AM Palmer Dabbelt [off-list ref] wrote:

quoted

On Tue, 30 Mar 2021 13:31:45 PDT (-0700), macro@orcam.me.uk wrote:

quoted

On Mon, 29 Mar 2021, Palmer Dabbelt wrote:

quoted

quoted

--- /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