linux - Some kernel ARM code -


i reading through arm kernel sources till stumbled upon following function :-

314 #define __get_user_asm_byte(x, addr, err)                       \ 315         __asm__ __volatile__(                                   \ 316         "1:     " tuser(ldrb) " %1,[%2],#0\n"                   \ 317         "2:\n"                                                  \ 318         "       .pushsection .fixup,\"ax\"\n"                   \ 319         "       .align  2\n"                                    \ 320         "3:     mov     %0, %3\n"                               \ 321         "       mov     %1, #0\n"                               \ 322         "       b       2b\n"                                   \ 323         "       .popsection\n"                                  \ 324         "       .pushsection __ex_table,\"a\"\n"                \ 325         "       .align  3\n"                                    \ 326         "       .long   1b, 3b\n"                               \ 327         "       .popsection"                                    \ 328         : "+r" (err), "=&r" (x)                                 \ 329         : "r" (addr), "i" (-efault)                             \ 330         : "cc")

the calling context seems follows :-

299 #define __get_user_err(x, ptr, err)                                     \ 300 {                                                                    \ 301         unsigned long __gu_addr = (unsigned long)(ptr);                 \ 302         unsigned long __gu_val;                                         \ 303         __chk_user_ptr(ptr);                                            \ 304         might_fault();                                                  \ 305         switch (sizeof(*(ptr))) {                                       \ 306         case 1: __get_user_asm_byte(__gu_val, __gu_addr, err);  break;  \

now have few doubts i'd clear arm assembly above.

  1. what function __get_user_asm_byte doing? can see r3 copied r0 , value 0 moved r1. after branches offset 0x2b?
  2. what function trying do? "+r" (err), "=&r" (x) line 328 onward mean?
  3. what .pushsection , .popsection?
  4. why arm assembly thats written kernel different syntactically(what's assembler used? why there %<regno> instead of r<regno>?)

first, comments have noted, syntax standard gcc inline assembly syntax (the +r, =&r, %<arg> parts).

the rest kernel magic designed handle page faults. point of get_user_asm_byte pull byte user-space. however, in pulling data user-space, 2 special situations need accommodated:

  1. a legitimate user-space address not present @ moment (i.e. because it's paged out)
  2. an illegal user-space address.

either cause page fault. (1), desired behavior restore user page (read in swap space or whatever), re-execute load instruction, resulting in eventual success; (2), desired behavior fail operation without retrying, returning efault error caller.

without special handling, normal behavior of page fault in kernel mode "oops". special sections coordinate page fault handling code , make possible recover correctly. if want understand details of how works, search __ex_table in kernel source.

also, checkout kernel doc at: documentation/x86/exception-tables.txt

the arm specific item use either ldrt or mmu domains; conditional in domain.h. modern arm cpus support domains. translated load/store variants apply user mode access older cpus.


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