From: Maciej W. Rozycki Date: Wed, 6 May 2026 22:42:23 +0000 (+0100) Subject: MIPS: DEC: Ensure 32-bit stack location for o32 prom_printf() X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5ff79e8bdc75db51e30298a75939e2308e7658e0;p=thirdparty%2Fkernel%2Flinux.git MIPS: DEC: Ensure 32-bit stack location for o32 prom_printf() In 64-bit configurations calling any firmware entry points from a kernel thread other than the initial one will result in a situation where the stack has been placed in the XKPHYS 64-bit memory segment. Consequently the stack pointer is no longer a 32-bit value and when the 32-bit firmware code called uses 32-bit ALU operations to manipulate the stack pointer, the calculated result is incorrect (in fact in the 64-bit MIPS ISA almost all 32-bit ALU operations will produce an unpredictable result when executed on 64-bit data) and control goes astray. This may happen when no final console driver has been enabled in the configuration and consequently the initial console continues being used late into bootstrap, or with an upcoming change that will switch the zs driver to use a platform device, which in turn will make the console handover happen only after other kernel threads have already been started, and the kernel will hang at: pid_max: default: 32768 minimum: 301 or somewhat later, but always before: cblist_init_generic: Setting adjustable number of callback queues. has been printed. It seems that only the prom_printf() entry point is affected. Of all the other entry points wired only rex_slot_address() and rex_gettcinfo() are called from a kernel thread other than the initial one, specifically kernel_init(), and they are leaf functions that do no business with the stack, having worked with no issue ever since 64-bit support was added for the platform back in 2002. To address this issue then, arrange for the stack to be switched in the o32 wrapper as required for prom_printf() only, by supplying call_o32() with a pointer to a chunk of initdata space, which is placed in the CKSEG0 32-bit compatibility segment, observing that prom_printf() is only called from console output handler and therefore with the console lock held, implying no need for this code to be reentrant. Other firmware entry points may be called with interrupts enabled and no lock held, and may therefore require that call_o32() be reentrant. They trigger no issue at this point and "if it ain't broke, don't fix it," so just leave them alone. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Maciej W. Rozycki Cc: stable@vger.kernel.org # v2.6.12+ Signed-off-by: Thomas Bogendoerfer --- diff --git a/arch/mips/dec/prom/init.c b/arch/mips/dec/prom/init.c index 8d74d7d6c05b4..e46c7acceffff 100644 --- a/arch/mips/dec/prom/init.c +++ b/arch/mips/dec/prom/init.c @@ -3,7 +3,7 @@ * init.c: PROM library initialisation code. * * Copyright (C) 1998 Harald Koerfgen - * Copyright (C) 2002, 2004 Maciej W. Rozycki + * Copyright (C) 2002, 2004, 2026 Maciej W. Rozycki */ #include #include @@ -20,6 +20,10 @@ #include +#ifdef CONFIG_64BIT +unsigned long o32_stk[O32_STK_SIZE] __initdata = { 0 }; +#endif + int (*__rex_bootinit)(void); int (*__rex_bootread)(void); int (*__rex_getbitmap)(memmap *); diff --git a/arch/mips/include/asm/dec/prom.h b/arch/mips/include/asm/dec/prom.h index 8fcad6984389c..af068e634f1e3 100644 --- a/arch/mips/include/asm/dec/prom.h +++ b/arch/mips/include/asm/dec/prom.h @@ -4,7 +4,7 @@ * * DECstation PROM interface. * - * Copyright (C) 2002 Maciej W. Rozycki + * Copyright (C) 2002, 2026 Maciej W. Rozycki * * Based on arch/mips/dec/prom/prom.h by the Anonymous. */ @@ -97,6 +97,17 @@ extern int (*__pmax_close)(int); #ifdef CONFIG_64BIT +#define O32_STK_SIZE 512 +extern unsigned long o32_stk[]; + +/* Switch the stack if outside the 32-bit address space. */ +static inline unsigned long *o32_get_stk(void) +{ + long fp = (long)__builtin_frame_address(0); + + return fp != (int)fp ? o32_stk + O32_STK_SIZE : NULL; +} + /* * On MIPS64 we have to call PROM functions via a helper * dispatcher to accommodate ABI incompatibilities. @@ -128,7 +139,7 @@ int __DEC_PROM_O32(_prom_printf, (int (*)(char *, ...), void *, char *, ...)); #define prom_getchar() _prom_getchar(__prom_getchar, NULL) #define prom_getenv(x) _prom_getenv(__prom_getenv, NULL, x) -#define prom_printf(x...) _prom_printf(__prom_printf, NULL, x) +#define prom_printf(x...) _prom_printf(__prom_printf, o32_get_stk(), x) #else /* !CONFIG_64BIT */