Hi Jim!
As you seem to already have understood, the first three line naively copies
'size' bytes from 'from' to 'to'. I write "naively" because it might happen
that the copy fails; then a memory exception would occur. A memory fault
could occur for several reasons having there own way to handle the fault.
The extra code used in '__copy_user' is one way to handle simple errors.
The kernel contains a table (in section '__ex_table') containing entries (X,
Y) saying that if an error occurs at address X, then jump to address Y. The
entries declared in the '__copy_user' code thus say that if a memory error
occurs when executing the code at the label '0:', the handler should return
to (and then execute) the code at the label '3:', calculating the actual
number of copied bytes (I think :-). Likewise, an error at '1:' makes the
handler return to '2:', just skipping any more copying.
The pseudoinstruction '.previous' just tells the assembler to put the
following code/data in the section used before the current section, probably
the '.text' section.
Labels like '0:', '1:', ... are "local labels", which can be used several
times for different locations in the same code. They are then referenced by
'0b', '1f', ... meaning "label 0 searching backwards" and "label 1 searching
forwards" respectively.
/Per
(By trying to explain something to someone else, you actually mostly explain
it to yourself.)
Split-up code:
#define __copy_user(to,from,size) \
do { \
int __d0, __d1; \
__asm__ __volatile__( \
Naive copying code:
"0: rep; movsl\n" \
" movl %3,%0\n" \
"1: rep; movsb\n" \
"2:\n" \
Fixup code:
".section .fixup,\"ax\"\n" \
"3: lea 0(%3,%0,4),%0\n" \
" jmp 2b\n" \
".previous\n" \
Exception table:
".section __ex_table,\"a\"\n" \
" .align 4\n" \
" .long 0b,3b\n" \
" .long 1b,2b\n" \
".previous" \
"Arguments":
: "=&c"(size), "=&D" (__d0), "=&S" (__d1) \
: "r"(size & 3), "0"(size / 4), "1"(to), "2"(from) \
: "memory"); \
} while (0)
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