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by Paul McJones last modified 2007-02-02 09:33

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/*ident	"@(#)cfront:patch/patch.c	1.3"*/
/*patch: Patch an a.out to ensure that static constructors are called.
Currently this is in good-old-C.

 This code uses -lld, the Unix system V functions for accessing COFF files.
	The program is passed one argument, the name of an executable
	C++ program.  It first reads the COFF symbol table, remembering
	all symbols of the name __LINK.  
	  Each of these symbols points to a structure of the form:
		struct __link {
			struct __link *next;	//next link in the chain
			int (*ctor)();		//ptr to ctor function
			int (*dtor)();		//ptr to dtor function
	cfront puts one of these in each dot-o.  Patch finds them,
	and chains them together by writing
	(into the actual a.out) values for the "next"
	pointers.  A pointer to the start of the chain is written
	into the struct __link pointer named __HEAD.
	 _main will follow this chain at runtime and 
	use the ctor and dtor function pointers to call the static
	ctors and dtors.

#include <stdio.h>
#include <filehdr.h>
#include  <scnhdr.h>
#include <syms.h>
#include <ldfcn.h>
typedef long PHSADDR;
/*char *ldgetname(LDFILE*, SYMENT*);*/
/*void fatal_error(char *);*/
char *ldgetname();
void fatal_error();
int debug = 0;
int found_main = 0;	/*1 iff _main() has been seen*/
PHSADDR head = 0;	/*Adress of the __HEAD symbol*/
PHSADDR data_infile;	/*offset in file of the data segment*/
PHSADDR data_pa;	/*physical adress of start of data segment*/
PHSADDR zero = 0;
PHSADDR previous;
char *file;			/*Name of the file being patched*/

#define MAX_FILES 5000
PHSADDR addrs[MAX_FILES];	/*array of addresses of __LINK symbols*/
PHSADDR *addr_ptr = addrs;

main(argc , argv)
int argc;
char *argv[];
	LDFILE *ldptr;
	SCNHDR secthead;
	FILE *fptr;
	PHSADDR *symbol;
	int x_sym;
	if (argc == 3 && strcmp(argv[1], "-d")==0 )
		file = argv[2];
		debug = 1;
	else if (argc == 2)
		file = argv[1];
		fatal_error("usage: patch file");
			/*Try to open the file*/
	if ( (ldptr = ldopen(file, NULL)) == NULL)
		fatal_error("cannot open file");
			/*Find  the beginning of the .data section*/
	if (ldnshread( ldptr, ".data", &secthead) == FAILURE)
		fatal_error("cannot get .data header");
			/*remember the start of data section, both the
			  physical address at runtime,
			  and the offset in the file*/
	data_infile = secthead.s_scnptr;
	data_pa = secthead.s_paddr;
	if (debug)
		printf("data in file: 0x%x\n", data_infile);
		printf("\taddr: 0x%x\n", data_pa);
			/*seek to the beginning of the symbol table*/
	if (ldtbseek(ldptr) == FAILURE)
		fatal_error("cannot get symbol table");

			/*Find the magic symbols in the a.out*/
        for ( x_sym = 0; x_sym < HEADER(ldptr).f_nsyms; ++x_sym)
		char *str;
		SYMENT sym;
                        /*read the symbol*/
		if ( FREAD( (char *)&sym, SYMESZ, 1, ldptr) != 1)
		str = ldgetname(ldptr,&sym);
		if (strcmp(str, "__HEAD") == 0)
			if (debug)
				printf("__HEAD found at 0x%x\n", 
			head = sym.n_value;
		else if (strcmp(str, "__LINK")==0)
			if (addr_ptr >= addrs + MAX_FILES)
				fatal_error(" too many files");
			*addr_ptr++ = sym.n_value;
			if (debug)
				printf("__LINK found at 0x%x\n",sym.n_value);
		else if (strcmp(str, "__main") == 0)

			/*Skip auxiliary entries*/
		if (sym.n_numaux)
                        x_sym += sym.n_numaux;
                        FSEEK(ldptr, sym.n_numaux*SYMESZ, 1);
	if (!head)
		if (found_main == 0)
			fatal_error("_main() not found");
		fatal_error("Bad _main loaded- libC probably not set up for patch");
			/*Now we have all of the __LINK pointers.
			  close the file, and reopen it for updating to
			  write the patches.  All hell will break loose
			  if someone writes the file in the meantime.*/
			/*If no symbols were found, quit*/
	if ( addr_ptr == addrs)
			printf("No __LINKs found\n");

	if ( (fptr=fopen(file, "r+")) == NULL)
		fatal_error(" can't reopen file");

			/*patch the first symbol*/
			/*seek to: physical adr. of symbol
				- physical adr of start of data
				+ file offset of start of data*/

	previous = head - data_pa + data_infile;

			/*Now, go thru the list of symbols. Each is
			  a pointer to the next link. Chain them up*/
			/*For non-obvious reasons, do this backwards.
			  This calls ctors from libraries first.*/

	for (symbol = addr_ptr - 1; symbol >= addrs; symbol --)
			/*Update the previous pointer to point to this one.*/
			printf("Write 0x%x at offset 0x%x\n",
			*symbol, previous );
		if (fseek(fptr, previous , 0))
			fatal_error("can't seek");
		if( fwrite((char *)symbol, sizeof(PHSADDR), 1, fptr) == 0)
			fatal_error("can't write file");
		previous = *symbol - data_pa + data_infile;
			/*Zero out the last symbol*/
	if(fseek(fptr, previous , 0))
		fatal_error("can't seek");
	if (fwrite((char *)&zero, sizeof(PHSADDR), 1, fptr) == 0)
		fatal_error("can't write");
		printf("Write 0 at offset 0x%x\n", previous );
char * message;
	fprintf(stderr,"patch: file %s: %s\n", file, message);
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