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

[Alexandre Ghiti]

On 3/3/23 17:40, Arnd Bergmann wrote:
> On Fri, Mar 3, 2023, at 12:59, Alexandre Ghiti wrote:
> > On 3/2/23 20:50, H. Peter Anvin wrote:
> > > On March 1, 2023 7:17:18 PM PST, Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:
> > > > > > 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?
> On x86, the COMMAND_LINE_SIZE value is already not part of a uapi header,
> but instead (since bzImage format version 2.06) is communicated from
> the kernel to the boot loader, which then knows how much data the
> kernel will read (at most) from the command line.
>
> Most x86 kernels these days are booted using UEFI, which I think has
> no such interface, the firmware just passes the command line and a
> length, but has no way of knowing if the kernel will truncate this.
> I think that is the same as with any other architecture that passes
> the command line through UEFI, DT or ATAGS, all of which use
> length/value pairs.
>
> Russell argued on IRC that this can be considered an ABI since a
> boot loader may use its knowledge of the kernel's command line size
> limit to reject long command lines. On the other hand, I don't
> think that any boot loader actually does, they just trust that it
> fits and don't have a good way of rejecting invalid configuration
> other than truncating and/or warning.
>
> One notable exception I found while looking through is the old
> (pre-ATAGS) parameter structure on Arm, which uses COMMAND_LINE_SIZE
> as part of the structure definition. Apparently this was deprecated
> 22 years ago, so hopefully the remaining riscpc and footbridge
> users have all upgraded their bootloaders.
>
> The only other case I could find that might go wrong is
> m68knommu with a few files copying a COMMAND_LINE_SIZE sized
> buffer from flash into a kernel buffer:
>
> arch/m68k/coldfire/m5206.c:void __init config_BSP(char *commandp, int size)
> arch/m68k/coldfire/m5206.c-{
> arch/m68k/coldfire/m5206.c-#if defined(CONFIG_NETtel)
> arch/m68k/coldfire/m5206.c-     /* Copy command line from FLASH to local buffer... */
> arch/m68k/coldfire/m5206.c-     memcpy(commandp, (char *) 0xf0004000, size);
> arch/m68k/coldfire/m5206.c-     commandp[size-1] = 0;
> arch/m68k/coldfire/m5206.c-#endif /* CONFIG_NETtel */


I see, thanks your thorough explanation: I don't see this m64k issue as 
a blocker (unless Geert disagrees but he already reviewed the m64k 
patches),  so I'll send the v5 now.

Thanks again,

Alex


>       Arnd