After a major OS upgrade this C code behaviour has changed:
...if ((fd = open(argv[1], O_RDWR | O_SYNC)) == -1) FATAL;printf("character device %s opened.\n", argv[1]);fflush(stdout);/* map one page */map_base = mmap(0xe0000000, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);if (map_base == (void *)-1) FATAL;printf("Memory mapped at address %p.\n", map_base);...With a binary inherited from an old OS, "old mmap" returns a virtual address 0x7fb20d725000. If I rebuild the same C file on a new OS, it returns 0xe0000000 which seems to be a physical, and subsequent code - which uses this returned address - now fails with a segmentation fault.
How to force mmap to work as before without downgrading the OS or using old binary? Any modern flags for gcc or mmap itself?
Run a code example below with sudo ./test /dev/zero 0x01000000 : (/dev/zero instead of a real device gives the same results)
#include <stdint.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <byteswap.h>#include <string.h>#include <errno.h>#include <signal.h>#include <fcntl.h>#include <ctype.h>#include <termios.h>#include <sys/types.h>#include <sys/mman.h>/* ltoh: little to host *//* htol: little to host */#if __BYTE_ORDER == __LITTLE_ENDIAN#define ltohl(x) (x)#define ltohs(x) (x)#define htoll(x) (x)#define htols(x) (x)#elif __BYTE_ORDER == __BIG_ENDIAN#define ltohl(x) __bswap_32(x)#define ltohs(x) __bswap_16(x)#define htoll(x) __bswap_32(x)#define htols(x) __bswap_16(x)#endif#define FATAL do { fprintf(stderr, "Error at line %d, file %s (%d) [%s]\n", __LINE__, __FILE__, errno, strerror(errno)); exit(1); } while(0)#define MAP_SIZE (16*1024*1024UL)#define MAP_MASK (MAP_SIZE - 1)int main(int argc, char **argv){ int fd; void *map_base, *virt_addr; uint32_t read_result, writeval; off_t target; char *device; if (argc != 3) { fprintf(stderr,"\nUsage:\t%s <device> <address> [[type] data]\n""\tdevice : character device to access\n""\taddress : memory address to access\n\n", argv[0]); exit(1); } device = strdup(argv[1]); target = strtoul(argv[2], 0, 0); fprintf("argc = %d, device: %s, address: 0x%08x\n", argc, device, (unsigned int)target); if ((fd = open(argv[1], O_RDWR | O_SYNC)) == -1) FATAL; fprintf(stdout, "character device %s opened.\n", argv[1]); fflush(stdout); /* map one page */ map_base = mmap(0xe0000000, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if (map_base == (void *)-1) FATAL; fprintf(stdout, "Memory mapped at address %p.\n", map_base); fflush(stdout); /* calculate the virtual address to be accessed */ virt_addr = map_base + target; /* read only */ read_result = *((uint32_t *) virt_addr); /* swap 32-bit endianess if host is not little-endian */ read_result = ltohl(read_result); printf("Read 32-bit value at address 0x%08x (%p): 0x%08x\n", (unsigned int)target, virt_addr, (unsigned int)read_result); if (munmap(map_base, MAP_SIZE) == -1) FATAL; close(fd); return 0;}