Re: [PATCH] MIPS: Fix big-endian stack argument fetching in o32 wrapper

[Thomas Bogendoerfer <tsbogend@alpha.franken.de>]

On Sat, May 02, 2026 at 12:14:20AM +0100, Maciej W. Rozycki wrote:
> Fix an issue in call_o32() where the upper 32-bit half of incoming n64 
> stack arguments is fetched and used for outgoing o32 stack arguments on 
> big-endian platforms.
> 
> This code was adapted from arch/mips/dec/prom/call_o32.S which was meant 
> for a little-endian platform only and therefore using 32-bit loads from 
> 64-bit stack slot locations holding incoming stack arguments resulted in 
> correct values being retrieved for data that is expected to be 32-bit.  
> 
> This works on little-endian platforms where the lower 32-bit half of the 
> 64-bit value is located at every 64-bit stack slot location.  However on 
> big-endian platforms the lower 32-bit half is instead located at offset 
> 4 from every 64-bit stack slot location.
> 
> So to fix the issue the offset of 4 would have to be used on big-endian 
> platforms only, or alternatively a 64-bit load from the 64-bit stack 
> slot location can be used across the board, as the subsequent 32-bit 
> store to the corresponding outgoing stack argument slot will correctly 
> truncate the value and cause no unpredictable result.  We already take 
> advantage of this architectural feature for the incoming arguments held 
> in $a6 and $a7 registers, since the o32 wrapper does not know how many 
> incoming arguments there are and consequently propagates incoming data 
> which may not be 32-bit.
> 
> Since this code is generally supposed to be used with the stack located 
> in cached memory there is no extra overhead expected for 64-bit loads as 
> opposed to 32-bit ones, so pick this variant for code simplicity.
> 
> Fixes: 231a35d37293 ("[MIPS] RM: Collected changes")
> Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
> ---
>  arch/mips/fw/lib/call_o32.S |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

applied to mips-next

Thomas.

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