Hi Peter, On 3/2/23 20:50, H. Peter Anvin wrote:
On March 1, 2023 7:17:18 PM PST, Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:On Tue, 14 Feb 2023 01:19:02 PST (-0800), hca@xxxxxxxxxxxxx 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@xxxxxxxxxxxxx> 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.)
Is COMMAND_LINE_SIZE what you call the default length? Does that mean that to you the patchset is wrong?
Thanks, Alex
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