# read: analogously to write, support reading from in-memory streams in # addition to file descriptors. # # We can pass it either a file descriptor or an address to a stream. If a # file descriptor is passed in, we _read from it using the right syscall. If a # stream is passed in (a fake file descriptor), we read from it instead. This # lets us initialize input for tests. # # A little counter-intuitively, the output of 'read' ends up in.. a stream. So # tests end up doing a redundant copy. Why? Well, consider the alternatives: # # a) Reading into a string, and returning a pointer to the end of the read # region, or a count of bytes written. Now this count or end pointer must be # managed separately by the caller, which can be error-prone. # # b) Having 'read' return a buffer that it allocates. But there's no way to # know in advance how large to make the buffer. If you read less than the # size of the buffer you again end up needing to manage initialized vs # uninitialized memory. # # c) Creating more helpful variants like 'read-byte' or 'read-until' which # also can take a file descriptor or stream, just like 'write'. But such # primitives don't exist in the Linux kernel, so we'd be implementing them # somehow, either with more internal buffering or by making multiple # syscalls. # # Reading into a stream avoids these problems. The buffer is externally # provided and the caller has control over where it's allocated, its lifetime, # and so on. The buffer's read and write pointers are internal to it so it's # easier to keep in a consistent state. And it can now be passed directly to # helpers like 'read-byte' or 'read-until' that only need to support streams, # never file descriptors. # # Like with 'write', we assume our data segment will never begin at an address # shorter than 0x08000000, so any smaller arguments are assumed to be real # file descriptors. # # As a reminder, a stream looks like this: # write: int # index at which to write to next # read: int # index at which to read next # data: (array byte) # prefixed by length as usual == code # instruction effective address register displacement immediate # . op subop mod rm32 base index scale r32 # . 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes # main: e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'. # syscall(exit, Num-test-failures) 8b/copy 0/mod/indirect 5/rm32/.disp32 . . 3/r32/EBX Num-test-failures/disp32 # copy *Num-test-failures to EBX b8/copy-to-EAX 1/imm32/exit cd/syscall 0x80/imm8 read: # f : fd or (address stream), s : (address stream) -> num-bytes-read/EAX # . prolog 55/push-EBP 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP # if (f < 0x08000000) return _read(f, s) # f can't be a user-mode address, so treat it as a kernel file descriptor 81 7/subop/compare 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 8/disp8 0x08000000/imm32 # compare *(EBP+8) 7d/jump-if-greater-or-equal $read:fake/disp8 # . . push args ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 0xc/disp8 . # push *(EBP+12) ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 8/disp8 . # push *(EBP+8) # . . call e8/call _read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # return eb/jump $read:end/disp8 $read:fake: # otherwise, treat 'f' as a stream to scan from # . save registers 56/push-ESI 57/push-EDI # ESI = f 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 6/r32/ESI 8/disp8 . # copy *(EBP+8) to ESI # EDI = s 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 7/r32/EDI 0xc/disp8 . # copy *(EBP+12) to ESI # EAX = _append-4(out = &s->data[s->write], outend = &s->data[s->length], # in = &f->data[f->read], inend = &f->data[f->write]) # . . push &f->data[f->write] 8b/copy 0/mod/indirect 6/rm32/ESI . . . 0/r32/EAX . . # copy *ESI to EAX 8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/ESI 0/index/EAX . 0/r32/EAX 0xc/disp8 . # copy ESI+EAX+12 to EAX 50/push-EAX # . . push &f->data[f->read] 8b/copy 1/mod/*+disp8 6/rm32/ESI . . . 0/r32/EAX 4/disp8 . # copy *(ESI+4) to EAX 8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/ESI 0/index/EAX . 0/r32/EAX 0xc/disp8 . # copy ESI+EAX+12 to EAX 50/push-EAX # . . push &s->data[s->length] 8b/copy 1/mod/*+disp8 7/rm32/EDI . . . 0/r32/EAX 8/disp8 . # copy *(EDI+8) to EAX 8d/copy-address 1/mod/*+disp8 4/rm32/sib 7/base/EDI 0/index/EAX . 0/r32/EAX 0xc/disp8 . # copy EDI+EAX+12 to EAX 50/push-EAX # . . push &s->data[s->write] 8b/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy *EDI to EAX 8d/copy-address 1/mod/*+disp8 4/rm32/sib 7/base/EDI 0/index/EAX . 0/r32/EAX 0xc/disp8 . # copy EDI+EAX+12 to EAX 50/push-EAX # . . call e8/call _append-4/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0x10/imm32 # add to ESP # s->write += EAX 01/add 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # add EAX to *EDI # f->read += EAX 01/add 1/mod/*+disp8 6/rm32/ESI . . . 0/r32/EAX 4/disp8 . # add EAX to *(ESI+4) # . restore registers 5f/pop-to-EDI 5e/pop-to-ESI $read:end: # . epilog 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 5d/pop-to-EBP c3/return # - helpers # idea: a clear-if-empty method on streams that clears only if f->read == f->write # Unclear how I'd use it, though. Callers seem to need the check anyway. # Maybe a better helper would be 'empty-stream?' _read: # fd : int, s : (address stream) -> num-bytes-read/EAX # . prolog 55/push-EBP 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP # . save registers 51/push-ECX 52/push-EDX 53/push-EBX 56/push-ESI # ESI = s 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 6/r32/ESI 0xc/disp8 . # copy *(EBP+12) to ESI # EAX = s->write 8b/copy 0/mod/indirect 6/rm32/ESI . . . 0/r32/EAX . . # copy *ESI to EAX # EDX = s->length 8b/copy 1/mod/*+disp8 6/rm32/ESI . . . 2/r32/EDX 8/disp8 . # copy *(ESI+8) to EDX # syscall(read, fd, &s->data[s->write], s->length - s->write) # . . fd : EBX 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 3/r32/EBX 8/disp8 . # copy *(EBP+8) to EBX # . . data : ECX = &s->data[s->write] 8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/ESI 0/index/EAX . 1/r32/ECX 0xc/disp8 . # copy ESI+EAX+12 to ECX # . . size : EDX = s->length - s->write 29/subtract 3/mod/direct 2/rm32/EDX . . . 0/r32/EAX . . # subtract EAX from EDX # . . syscall b8/copy-to-EAX 3/imm32/read cd/syscall 0x80/imm8 # add the result EAX to s->write 01/add 0/mod/indirect 6/rm32/ESI . . . 0/r32/EAX . . # add EAX to *ESI $_read:end: # . restore registers 5e/pop-to-ESI 5b/pop-to-EBX 5a/pop-to-EDX 59/pop-to-ECX # . epilog 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 5d/pop-to-EBP c3/return # Two options: # 1 (what we have above): # ECX = s # EAX = s->write # EDX = s->length # # syscall # ECX = lea ECX+EAX+12 # EDX = sub EDX EAX # # 2: # ECX = s # EDX = s->length # ECX = &s->data # # syscall # ECX = add ECX, s->write # EDX = sub EDX, s->write # # Not much to choose between the two? Option 2 performs a duplicate load to # use one less register, but doesn't increase the amount of spilling (ECX # and EDX must be used, and EAX must be clobbered anyway). # - tests test-read-single: # - write a single character into _test-stream, then read from its buffered-file # clear-stream(_test-stream) # . . push args 68/push _test-stream/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # clear-stream(_test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # write(_test-stream, "Ab") # . . push args 68/push "Ab"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # read(_test-stream, _test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # check-ints-equal(EAX, 2, msg) # . . push args 68/push "F - test-read-single: return EAX"/imm32 68/push 2/imm32 50/push-EAX # . . call e8/call check-ints-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP # check-ints-equal(*_test-stream-buffer->data, 41/A 62/b 00 00, msg) # . . push args 68/push "F - test-read-single"/imm32 68/push 0x006241/imm32/Ab # . . push *_test-stream-buffer->data b8/copy-to-EAX _test-stream-buffer/imm32 ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 0xc/disp8 . # push *(EAX+12) # . . call e8/call check-ints-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP # end c3/return test-read-is-stateful: # - make two consecutive reads, check that their results are appended # clear-stream(_test-stream) # . . push args 68/push _test-stream/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # clear-stream(_test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # write(_test-stream, "C") # . . push args 68/push "C"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # read(_test-stream, _test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # write(_test-stream, "D") # . . push args 68/push "D"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # read(_test-stream, _test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # check-ints-equal(*_test-stream-buffer->data, 43/C 44/D 00 00, msg) # . . push args 68/push "F - test-read-is-stateful"/imm32 68/push 0x00004443/imm32/C-D # . . push *_test-stream-buffer->data b8/copy-to-EAX _test-stream-buffer/imm32 ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 0xc/disp8 . # push *(EAX+12) # . . call e8/call check-ints-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP # end c3/return test-read-returns-0-on-end-of-file: # - read after hitting end-of-file, check that result is 0 # setup # . clear-stream(_test-stream) # . . push args 68/push _test-stream/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # . clear-stream(_test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP # . write(_test-stream, "Ab") # . . push args 68/push "Ab"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # first read gets to end-of-file # . read(_test-stream, _test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # second read # . read(_test-stream, _test-stream-buffer) # . . push args 68/push _test-stream-buffer/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP # check-ints-equal(EAX, 0, msg) # . . push args 68/push "F - test-read-returns-0-on-end-of-file"/imm32 68/push 0/imm32 50/push-EAX # . . call e8/call check-ints-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP # end c3/return == data _test-stream-buffer: # current write index 00 00 00 00 # current read index 00 00 00 00 # length (= 8) 08 00 00 00 # data 00 00 00 00 00 00 00 00 # 8 bytes # . . vim:nowrap:textwidth=0