QEMU's 9pfs readonly check at line 2108-2109 tests mode & O_TRUNC and mode & O_APPEND against the raw 9P open mode byte. For the 9P2000.u protocol, mode uses 9P flag values (Otrunc=0x10, Oappend=0x80), but the check compares against Linux constants (O_TRUNC=0x200, O_APPEND=0x400). Since 0x10 & 0x200 = 0 and 0x80 & 0x400 = 0, both flags bypass the readonly guard. The mode is then correctly converted to host flags via omode_to_uflags(), so open() is called with O_TRUNC -- truncating host files on readonly shares.
9P2000.L is NOT affected (guest passes Linux flags directly, so the bit values match).