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April 2026

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stack overflow/wraparound in 64 bit powerpc openfirmware
by Adrian Chadd April 15, 2026

April 15, 2026

hi! I spent some time debugging why FreeBSD isn't booting on qemu+mac99+openbios openfirmware. I filed an issue (https://github.com/openbios/openbios/issues/27) and a pull request (https://github.com/openbios/openbios/pull/28) . The TL;DR - SAVE_SPACE isn't enough to store all the registers - r29, r30, r31 wrap around to address 0x0 which corrupts that memory. It doesn't crash in early boot because FreeBSD hasn't yet remapped the page tables to memory mapped at 0x0 but once it does, the next openfirmware call traps. The fix is pretty simple: diff --git a/arch/ppc/qemu/start.S b/arch/ppc/qemu/start.S index c679230..4f4678f 100644 --- a/arch/ppc/qemu/start.S +++ b/arch/ppc/qemu/start.S @@ -503,7 +503,7 @@ _GLOBAL(saved_stack): #define SAVE_SPACE 320 #else #define STKOFF 8 -#define SAVE_SPACE 144 +#define SAVE_SPACE 156 #endif GLOBL(of_client_callback): Thanks! -adrian

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