JMP instruction - Hex code
Solution 1
There is no jump of the form JMP absaddr
to an absolute address in 64 bit mode. The operand of a jump is always a 32 bit relative displacement to rip
, which gets sign extended to 64 bit.
The reason you see no consistency is possibly that the offset depends on the the current instruction pointer and you didn't recognize that.
jmp eax
isn't allowed either, as addresses are of course always 64 bit wide on a 64 bit architecture. A sequence mov rax, addr + jmp rax
is possible, it would look like
48 c7 c0 35 08 40 00 mov rax, 0x00400835
ff e0 jmp rax
or
48 b8 35 08 40 00 00 00 00 00 mov rax, 0x0000000000400835
ff e0 jmp rax
How did I know these hex codes? Well, I did ask my compiler. I compiled with gcc -c
and disassembled with objdump
. I didn't bother to use Intel syntax, because I don't need it. So this is in AT&T syntax.
echo 'asm("mov $400835, %rax\n jmp *%rax\n");' > test.c
gcc -c test.c
objdump -d test.o
Solution 2
If you don't want to use a register for whatever reason, it's also possible to encode a 64 bit absolute immediate jump as
ff 25 00 00 00 00 jmp qword ptr [rip] jmp *(%rip)
yo ur ad dr re ss he re some random assembly
rip refers to the instruction pointer AFTER the jmp instruction itself, so it's a pointer to your address.
Deepanjan Mazumdar
Updated on July 09, 2022Comments
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Deepanjan Mazumdar almost 2 years
Have a doubt regarding the hex code conversion of JMP machine instruction. I have the absolute address I want to jump to, say "JMP 0x400835". First of all, is this allowed? If yes, what would be the corresponding hex code? If not, can I first store the address in some register, say EAX and then put "JMP EAX"? I am working on x86(64b) architecture.
I have tried to print out the hex code from the diassem output in gdb, but there is no consistency, ie, I do not see the destination address in the hex code.
I am new to hex code and machine instructions, so pardon my ignorance.
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Deepanjan Mazumdar about 12 yearsThank you for the answer. This really helped me. objdump is seriously a good tool!
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Deepanjan Mazumdar about 12 yearsHey.. I have marked your post as "useful".. that's what is needed I guess, right?
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Man of One Way almost 10 yearsDo you mean "jmp absaddr" will be "jmp %rip + absaddr" (where absaddr has to be equal or less than 32bit)?
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Gunther Piez almost 10 years@ManofOneWay Nor sure what you are trying to ask, as there is no
jmp absaddr
andjmp %rip+absaddr
isn't absolute. If you tell the assembler to translate some code in the kind ofjmp someaddr
it will calculate the relative distance from the current address (which will later on execution time be in%rip
) tosomeaddr
and emite9 xx xx xx xx
, where the four xx bytes correspond to the calculated address difference. -
Kibernetik over 9 yearsHere you write "jmp *%rax". Can you please tell where I can find description of this syntax? Because everywhere AT&T syntax is explained, this JMP syntax is not listed.
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Peter Cordes over 5 years
mov eax, 0x00400835
/mov $0x00400835, %eax
is only 5 bytes. There's zero benefit to sign-extending that immediate with a 64-bitmov
instead of relying on implicit zero-extension from writing EAX. Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?