Why doesn't gcc support naked functions?

Question

I use naked functions to patch parts of a program while it's running. I can easily do this in VC++ in Windows. I'm trying to do this in Linux and it seems gcc doesn't support naked functions. Compiling code with naked functions gives me this: warning: 'naked' attribute directive ignored. Compiled under CentOS 5.5 i386.

Answer 1

The naked attribute is only supported by GCC on certain platforms (ARM, AVR, MCORE, RX and SPU) according to the docs :

naked : Use this attribute on the ARM, AVR, MCORE, RX and SPU ports to indicate that the specified function does not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences. The only statements that can be safely included in naked functions are asm statements that do not have operands. All other statements, including declarations of local variables, if statements, and so forth, should be avoided. Naked functions should be used to implement the body of an assembly function, while allowing the compiler to construct the requisite function declaration for the assembler.

I'm not sure why.

Answer 2

On x86 you can workaround by using asm at global scope instead:

int write(int fd, const void *buf, int count);                                            

asm                                                                              
(                                                                                
".global write                             \n\t"                                    
"write:                                    \n\t"
"       pusha                              \n\t"                                    
"       movl   $4, %eax                    \n\t"                                    
"       movl   36(%esp), %ebx              \n\t"                                    
"       movl   40(%esp), %ecx              \n\t"                                    
"       movl   44(%esp), %edx              \n\t"                                    
"       int    $0x80                       \n\t"                                    
"       popa                               \n\t"                                    
"       ret                                \n\t"                                    
);                                                                               

void _start()                                                                    
{                                                                                
#define w(x) write(1, x, sizeof(x));                                             
    w("hello\n");                                                                
    w("bye\n");                                                                  
}                                                                                

Also naked is listed among x86 function attributes , so I suppose it works for newer gcc.

Answer 3

GCC only supports naked functions on ARM and other embedded platforms. http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

Also, what you're doing is inherently unsafe, as you cannot guarantee that the code you're patching isn't executing if the program is running.

Answer 4

That's an ugly solution. Link against a .asm file for your target architecture.