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# 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 size 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
read: # f: fd or (addr stream byte), s: (addr stream byte) -> num-bytes-read/eax: int
# . prologue
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 5/rm32/ebp . . . . 8/disp8 0x08000000/imm32 # compare *(ebp+8)
73/jump-if-addr>= $read:fake/disp8
# . . push args
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12)
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 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 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi
# edi = s
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 0xc/disp8 . # copy *(ebp+12) to esi
# eax = _buffer-4(out = &s->data[s->write], outend = &s->data[s->size],
# 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->size]
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 _buffer-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:
# . epilogue
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
# - helpers
# '_buffer' is like '_append', but silently stops instead of aborting when it runs out of space
# 3-argument variant of _buffer
_buffer-3: # out: address, outend: address, s: (array byte) -> num_bytes_buffered/eax
# . prologue
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# . save registers
51/push-ecx
# eax = _buffer-4(out, outend, &s->data[0], &s->data[s->size])
# . . push &s->data[s->size]
8b/copy 1/mod/*+disp8 5/rm32/ebp . . 0/r32/eax 0x10/disp8 . # copy *(ebp+16) to eax
8b/copy 0/mod/indirect 0/rm32/eax . . . 1/r32/ecx . . # copy *eax to ecx
8d/copy-address 1/mod/*+disp8 4/rm32/sib 0/base/eax 1/index/ecx . 1/r32/ecx 4/disp8 . # copy eax+ecx+4 to ecx
51/push-ecx
# . . push &s->data[0]
8d/copy-address 1/mod/*+disp8 0/rm32/eax . . . 1/r32/ecx 4/disp8 . # copy eax+4 to ecx
51/push-ecx
# . . push outend
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12)
# . . push out
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
# . . call
e8/call _buffer-4/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0x10/imm32 # add to esp
$_buffer-3:end:
# . restore registers
59/pop-to-ecx
# . epilogue
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
# 4-argument variant of _buffer
_buffer-4: # out: address, outend: address, in: address, inend: address -> num_bytes_buffered/eax
# . prologue
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
57/push-edi
# eax/num_bytes_buffered = 0
b8/copy-to-eax 0/imm32
# edi = out
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 8/disp8 . # copy *(ebp+8) to edi
# edx = outend
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx
# esi = in
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 0x10/disp8 . # copy *(ebp+16) to esi
# ecx = inend
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 1/r32/ecx 0x14/disp8 . # copy *(ebp+20) to ecx
$_buffer-4:loop:
# if (in >= inend) break
39/compare 3/mod/direct 6/rm32/esi . . . 1/r32/ecx . . # compare esi with ecx
73/jump-if-addr>= $_buffer-4:end/disp8
# if (out >= outend) break # for now silently ignore filled up buffer
39/compare 3/mod/direct 7/rm32/edi . . . 2/r32/edx . . # compare edi with edx
73/jump-if-addr>= $_buffer-4:end/disp8
# *out = *in
8a/copy-byte 0/mod/indirect 6/rm32/esi . . . 3/r32/BL . . # copy byte at *esi to BL
88/copy-byte 0/mod/indirect 7/rm32/edi . . . 3/r32/BL . . # copy byte at BL to *edi
# ++num_bytes_buffered
40/increment-eax
# ++in
46/increment-esi
# ++out
47/increment-edi
eb/jump $_buffer-4:loop/disp8
$_buffer-4:end:
# . restore registers
5f/pop-to-edi
5e/pop-to-esi
5b/pop-to-ebx
5a/pop-to-edx
59/pop-to-ecx
# . epilogue
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
# 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: (addr stream byte) -> num-bytes-read/eax: int
# . prologue
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 5/rm32/ebp . . . 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->size
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->size - s->write)
# . . fd: ebx
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 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->size - s->write
29/subtract 3/mod/direct 2/rm32/edx . . . 0/r32/eax . . # subtract eax from edx
# . . syscall
e8/call syscall_read/disp32
# 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
# . epilogue
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->size
# # syscall
# ecx = lea ecx+eax+12
# edx = sub edx eax
#
# 2:
# ecx = s
# edx = s->size
# 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 it
# 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-tmp-stream)
# . . push args
68/push _test-tmp-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
# 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
# eax = read(_test-stream, _test-tmp-stream)
# . . push args
68/push _test-tmp-stream/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 value"/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-stream-equal(_test-tmp-stream, "Ab", msg)
# . . push args
68/push "F - test-read-single"/imm32
68/push "Ab"/imm32
68/push _test-tmp-stream/imm32
# . . call
e8/call check-stream-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-tmp-stream)
# . . push args
68/push _test-tmp-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
# 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-tmp-stream)
# . . push args
68/push _test-tmp-stream/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-tmp-stream)
# . . push args
68/push _test-tmp-stream/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-stream-equal(_test-tmp-stream, "CD", msg)
# . . push args
68/push "F - test-read-is-stateful"/imm32
68/push "CD"/imm32
68/push _test-tmp-stream/imm32
# . . call
e8/call check-stream-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-tmp-stream)
# . . push args
68/push _test-tmp-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
# . 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-tmp-stream)
# . . push args
68/push _test-tmp-stream/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-tmp-stream)
# . . push args
68/push _test-tmp-stream/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-tmp-stream: # (stream byte)
# current write index
0/imm32
# current read index
0/imm32
# size
8/imm32
# data
00 00 00 00 00 00 00 00 # 8 bytes
# . . vim:nowrap:textwidth=0
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