Hello everyone,
I am trying the self-modifying-code check of Valgrind but I am not sure if
I understand the definition of self-modifying-code in Valgrind correctly.
I had prepared an example (see below) that has function f1 that is first
executed in main, outputs something (number 4660). Afterward, two
instructions of f1 are modified, and f1 is then executed one more time. It
then outputs something (number 22068) that is different from the first
time.
When I run Valgrind with --smc-check=all and --smc-check=none I do not see
any difference in the outputs of Valgrind e.g., both times f1 produces
different numbers (e.g., self-modifying-code successfully runs despite the
--smc-check if turned on or off)
*Could someone please let me know if this behavior is expected from
Valgrind?*
I further looked into the source code and found
valgrind\VEX\priv\guest_generic_bb_to_IR.c that generates the code to
check. However, I do not know where such a check is executed. It would be
great if somebody knows where such a check takes place, and *where we can
modify the source code to just simply say e.g., self-modifying-code is
found*.
Thank you very much in advance.
Best regards,
Duc
=============================
Self-modifying-code example
-------
*#include <stdio.h>#include <sys/mman.h>#include <unistd.h>__asm__( ".text"
);__asm__( ".align 4096" );void f1( void ){ printf( "%d\n", 0x1234 );}void
f2( void ){ printf("this is just a dummy function");}int main( void ){
int rc; int pagesize; char *p; int i; printf( "f1=0x%08X.\n", f1 );
f1( ); pagesize = sysconf( _SC_PAGE_SIZE ); printf( "pagesize=%d
(0x%08X).\n", pagesize, pagesize ); if( pagesize == -1 ) return( 2 );
p = (char*) f1; rc = mprotect( p, pagesize, PROT_READ | PROT_WRITE |
PROT_EXEC ); printf( "rc=%d.\n", rc ); if( rc != 0 ) return( 2 );
printf( "'mprotect()' succeeded.\n" ); for( i = 0; i+1 < (size_t) f2-
(size_t)f1; i++ ) { if( ((char*) f1)[ i ] == 0x34 && ((char*) f1)[ i+1
] == 0x12 ) { ((char*) f1)[ i+1 ] =0x78;//here performs
self-modifying-code ((char*) f1)[ i+1 ] =0x56;//here performs
self-modifying-code } } f1( );//here the output of f1 will be
different from the first f1() call printf( "Call succeeded.\n" ); return(
0 );} *
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