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Properly create BLX instruction

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Rethinked and revised this function.
This previous one worked by "accident".
More a sheer luck of offsets on current firmware (as of writing)
But now, this is more mathematically correct way of creating a
THUMB -> ARM BLX instruction.
Also setting the new op in a set of 2 u16,
because patching code is in thumb.
This commit is contained in:
luigoalma 2019-06-29 14:37:09 +01:00
parent d4e1da0174
commit 76dbdf45cf
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GPG Key ID: 1155216FF5495A42
1 changed files with 9 additions and 3 deletions
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12
arm9/source/patches.c
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@ -555,18 +555,24 @@ u32 patchP9AMTicketWrapperZeroKeyIV(u8 *pos, u32 size, u32 firmVersion)
static const u8 pattern[] = {0x20, 0x21, 0xA6, 0xA8};
u32 function = (u32)memsearch(pos, __rt_memclr_pattern, size, sizeof(__rt_memclr_pattern));
u32 *off = (u32 *)memsearch(pos, pattern, size, sizeof(pattern));
u16 *off = (u16 *)memsearch(pos, pattern, size, sizeof(pattern));
if(function == 0 || off == NULL) return firmVersion == 0xFFFFFFFF ? 0 : 1;
s32 opjumpdistance = (s32)(function - ((u32)&off[2])) / 2;
//After the found code it's a BL call (at &off[2]), that will be replaced
//From Thumb, op distance for setting in BLX can be got with,
//(Destination_offset - blx_op_offset+2) / 2
s32 opjumpdistance = (s32)(function - ((u32)&off[3])) / 2;
//Beyond limit
if(opjumpdistance < -0x1fffff || opjumpdistance > 0x1fffff) return 1;
//r0 and r1 for old call are already correct for this one
//BLX __rt_memclr
off[1] = 0xE800F000U | (((u32)opjumpdistance & 0x7FF) << 16) | (((u32)opjumpdistance >> 11) & 0x3FF) | (((u32)opjumpdistance >> 21) & 0x400);
u32 op = (0xE800F000U | (((u32)opjumpdistance & 0x7FF) << 16) | (((u32)opjumpdistance >> 11) & 0x3FF) | (((u32)opjumpdistance >> 21) & 0x400)) & ~(1<<16);
off[2] = op;
off[3] = op >> 16;
return 0;
}