Intricacies of C Semantics

1. What are "strings"? char *p = "hello"; versus char q[] = "hello"; .
2. Address relationships int x; int y;
3. Word alignment
4. Standard Library: libc strlen strcpy strcat
5. Files in ./modret/

These notes are triggered by our attempt at writing a new Linux kernel module.

1. The C language static is like private of C++; that symbol is not exported.
2. Do not leave the sys-call-table writeable until the module exits. Not good. As soon as the redirect/hijack is made, bring it to read-only.
3. The following was found in a blog.

   struct linux_dirent64 *cur = dirp;
   ...
   int reclen = cur->d_reclen;
   char *next_rec = (char *)cur + reclen;
   int len = (int)dirp + rtn - (int)next_rec;
   

4. Bad C usage. Reckless int-long-ptr synonyms. The type int is compiler specific. sizeof(int) is typically 4 or 8 bytes. On very old systems, and even modern embedded systems, it can be 2 bytes. The width of a ptr is architecture specific. A ptr value on a 64-bit system is 8 bytes wide. The GNU C compiler indeed generated a warning about (int) dirp.
5. C ptr arithmetic. The resulting value of p + x is computed as the equivalent long long int value of ptr p, plus (ordinary arith) x * sizeof(p)
6. asmlinkage int (*ogetdents64) (unsigned int fd, struct linux_dirent64 *dirp, unsigned int count); should not have asmlinkage And, this is declaring a ptr variable named ogetdents64. It also declares that (i) the ptr it holds is the address of a function, (ii) this function takes three arguments as declared.
7. I was expecting the GNU C compiler to produce a warning/error on ogetdents64(...) versus (*ogetdents64)(...) But, it did not! TBD Further investigation is warranted.
8. What is the proper declaration of sys-call-table? void * * sys_call_table works expediently, but not "correct". TBD Will post details on this later.
9. sys_call_table initialization

#define SYSCALLTBLPM 0xffffffff81801680 /* pmateti 3.19.0-20-lowlatency  */
#define SYSCALLTBLAS 0xffffffff81801400 /* asish 4.0.1 */
#define __NR_ni 7 /* NR of sys_ni_syscall */