gcc: Speify address for a function while compiling

raghu2383 · 11-18-2008, 03:46 AM

Hi,

I am working on security topics, and I need to write a C file that has
only one function in it (without the main routine). I need to compile
it.
Normally the compilation commands for it would be gcc -c blank.c -o
blank.o This crates a .o file starting at address 0 with just the single
routine inside the .o file. My question is whether I can make the
routine be given a specific address, say for example 8048000 instead of
starting at location 0.
Thanks in advance.

matsp · 11-18-2008, 03:50 AM

You can do that by setting the right parameters for "ld" when you link it to form an executable. gcc doesn't have any clue about absolute addresses in the code.

--
Mats

hauzer · 11-18-2008, 07:51 AM

It's -e func specifically.

matsp · 11-18-2008, 07:55 AM

Quote:

Originally Posted by hauzer

It's -e func specifically.

That tells the linker you want to start in func, but not what the original post asked for, which is that the code should be located at a particular address.

Edit: To locate the code at a specific address, "ld -Ttext org ... " is the command to use, where org is the origin. I'm not sure if you can actually specify this to any byte address - I'm pretty sure that the OS loads a page at a time, so even if it's a precise address, "funny things" may happen with an address that is not precisely on a 4KB boundary.

--
Mats

slingerland3g · 11-18-2008, 10:05 AM

8048000 is the start of most Linux distros for the stack. I would not hard compile that either as that would not be portable. You can also see this within your /proc dirctory as well during alive session of the program or within gdb for stack analysis. Funny that the book 'Self-Service Linux' talks about this a bit.

matsp · 11-18-2008, 10:08 AM

Quote:

Originally Posted by slingerland3g

8048000 is the start of most Linux distros for the stack. I would not hard compile that either as that would not be portable. You can also see this within your /proc dirctory as well during alive session of the program or within gdb for stack analysis. Funny that the book 'Self-Service Linux' talks about this a bit.

Really. Most Linux processes I've looked at have the stack around 0xBFFF0000.

--
Mats

slingerland3g · 11-18-2008, 10:18 AM

For local variables that is correct. The 0804 addressess range would be the code segment.

matsp · 11-18-2008, 10:28 AM

Quote:

Originally Posted by slingerland3g

For local variables that is correct. The 0804 addressess range would be the code segment.

Right, that makes more sense.

--
Mats

hauzer · 11-18-2008, 11:11 AM

Quote:

Originally Posted by matsp

That tells the linker you want to start in func, but not what the original post asked for, which is that the code should be located at a particular address.

--
Mats

Ah, yes.

itCbitC · 11-18-2008, 12:40 PM

Quote:

Originally Posted by raghu2383

Hi,

I am working on security topics, and I need to write a C file that has
only one function in it (without the main routine). I need to compile
it.
Normally the compilation commands for it would be gcc -c blank.c -o
blank.o This crates a .o file starting at address 0 with just the single
routine inside the .o file. My question is whether I can make the
routine be given a specific address, say for example 8048000 instead of
starting at location 0.
Thanks in advance.

Why would you want to do something like that in the first place?

matsp · 11-19-2008, 03:28 AM

Quote:

Originally Posted by itCbitC

Why would you want to do something like that in the first place?

Usually to inject code into an existing application, I would say. Whether that is allowed to be discussed on this forum is another interesting aspect.

--
Mats

11-18-2008, 03:46 AM	#1
raghu2383 Registered User Join Date: Jun 2008 Posts: 19	gcc: Speify address for a function while compiling Hi, I am working on security topics, and I need to write a C file that has only one function in it (without the main routine). I need to compile it. Normally the compilation commands for it would be gcc -c blank.c -o blank.o This crates a .o file starting at address 0 with just the single routine inside the .o file. My question is whether I can make the routine be given a specific address, say for example 8048000 instead of starting at location 0. Thanks in advance.
raghu2383 is offline

11-18-2008, 03:50 AM	#2
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	You can do that by setting the right parameters for "ld" when you link it to form an executable. gcc doesn't have any clue about absolute addresses in the code. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
matsp is offline

11-18-2008, 10:05 AM	#5
slingerland3g Registered User Join Date: Jan 2008 Location: Seattle Posts: 476	8048000 is the start of most Linux distros for the stack. I would not hard compile that either as that would not be portable. You can also see this within your /proc dirctory as well during alive session of the program or within gdb for stack analysis. Funny that the book 'Self-Service Linux' talks about this a bit.
slingerland3g is offline

11-18-2008, 10:18 AM	#7
slingerland3g Registered User Join Date: Jan 2008 Location: Seattle Posts: 476	For local variables that is correct. The 0804 addressess range would be the code segment.
slingerland3g is offline

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