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4520 - several syscalls for files
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@akkartik
akkartik committed Aug 14, 2018
1 parent fd7198b commit 5cec03b
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3 changes: 3 additions & 0 deletions subx/010---vm.cc
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Expand Up @@ -120,6 +120,9 @@ inline uint8_t* mem_addr_u8(uint32_t addr) {
inline int8_t* mem_addr_i8(uint32_t addr) {
return reinterpret_cast<int8_t*>(&Mem.at(addr-Mem_offset));
}
inline char* mem_addr_string(uint32_t addr) {
return reinterpret_cast<char*>(&Mem.at(addr-Mem_offset));
}
inline uint32_t* mem_addr_u32(uint32_t addr) {
return reinterpret_cast<uint32_t*>(&Mem.at(addr-Mem_offset));
}
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78 changes: 78 additions & 0 deletions subx/020syscalls.cc
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Expand Up @@ -21,12 +21,90 @@ void process_int80() {
exit(/*exit code*/Reg[EBX].u);
break;
case 3:
trace(91, "run") << "read: " << Reg[EBX].u << ' ' << Reg[ECX].u << '/' << mem_addr_string(Reg[ECX].u) << ' ' << Reg[EDX].u << end();
Reg[EAX].i = read(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
trace(91, "run") << "result: " << Reg[EAX].i << end();
if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
break;
case 4:
trace(91, "run") << "write: " << Reg[EBX].u << ' ' << Reg[ECX].u << '/' << mem_addr_string(Reg[ECX].u) << ' ' << Reg[EDX].u << end();
Reg[EAX].i = write(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
trace(91, "run") << "result: " << Reg[EAX].i << end();
if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
break;
case 5: {
check_flags(ECX);
check_mode(EDX);
trace(91, "run") << "open: " << Reg[EBX].u << '/' << mem_addr_string(Reg[EBX].u) << ' ' << Reg[ECX].u << end();
Reg[EAX].i = open(/*filename*/mem_addr_string(Reg[EBX].u), /*flags*/Reg[ECX].u, /*mode*/0640);
trace(91, "run") << "result: " << Reg[EAX].i << end();
if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
break;
}
case 6:
trace(91, "run") << "close: " << Reg[EBX].u << end();
Reg[EAX].i = close(/*file descriptor*/Reg[EBX].u);
trace(91, "run") << "result: " << Reg[EAX].i << end();
if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
break;
case 8:
check_mode(ECX);
trace(91, "run") << "creat: " << Reg[EBX].u << '/' << mem_addr_string(Reg[EBX].u) << end();
Reg[EAX].i = creat(/*filename*/mem_addr_string(Reg[EBX].u), /*mode*/0640);
trace(91, "run") << "result: " << Reg[EAX].i << end();
if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
break;
case 10:
trace(91, "run") << "unlink: " << Reg[EBX].u << '/' << mem_addr_string(Reg[EBX].u) << end();
Reg[EAX].i = unlink(/*filename*/mem_addr_string(Reg[EBX].u));
trace(91, "run") << "result: " << Reg[EAX].i << end();
if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
break;
case 38:
trace(91, "run") << "rename: " << Reg[EBX].u << '/' << mem_addr_string(Reg[EBX].u) << " -> " << Reg[ECX].u << '/' << mem_addr_string(Reg[ECX].u) << end();
Reg[EAX].i = rename(/*old filename*/mem_addr_string(Reg[EBX].u), /*new filename*/mem_addr_string(Reg[ECX].u));
trace(91, "run") << "result: " << Reg[EAX].i << end();
if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
break;
case 45: // brk: modify size of data segment
trace(91, "run") << "grow data segment to " << Reg[EBX].u << end();
resize_mem(/*new end address*/Reg[EBX].u);
break;
default:
raise << HEXWORD << EIP << ": unimplemented syscall " << Reg[EAX].u << '\n' << end();
}
}

// SubX is oblivious to file permissions, directories, symbolic links, terminals, and much else besides.
// Also ignoring any concurrency considerations for now.
void check_flags(int reg) {
uint32_t flags = Reg[reg].u;
if (flags != ((flags & O_RDONLY) | (flags & O_WRONLY))) {
raise << HEXWORD << EIP << ": most POSIX flags to the open() syscall are not supported. Just O_RDONLY and O_WRONLY for now. Zero concurrent access support.\n" << end();
exit(1);
}
if ((flags & O_RDONLY) && (flags & O_WRONLY)) {
raise << HEXWORD << EIP << ": can't open a file for both reading and writing at once. See http://man7.org/linux/man-pages/man2/open.2.html.\n" << end();
exit(1);
}
}

void check_mode(int reg) {
if (Reg[reg].u != 0600) {
raise << HEXWORD << EIP << ": SubX is oblivious to file permissions; register " << reg << " must be 0.\n" << end();
exit(1);
}
}

void resize_mem(uint32_t new_end_address) {
if (new_end_address < Mem_offset) {
raise << HEXWORD << EIP << ": can't shrink data segment to before code segment\n";
return;
}
int32_t new_size = new_end_address - Mem_offset;
if (new_size < SIZE(Mem)) {
raise << HEXWORD << EIP << ": shrinking data segment is not supported.\n" << end();
return;
}
Mem.resize(new_size); // will throw exception on failure
}
Binary file added subx/ex8
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99 changes: 99 additions & 0 deletions subx/ex8.subx
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@@ -0,0 +1,99 @@
## example showing file syscalls
# Create a file, open it for writing, write a character to it, close it, open
# it for reading, read a character from it, close it, delete it, and return
# the character read.
#
# To run:
# $ subx translate ex8.subx ex8
# $ subx run ex8
# Expected result:
# $ echo $?
# 97

== 0x08048074 # code segment, after leaving room for ELF header and segment headers
# instruction effective address operand 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

## creat(filename)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b9/copy . . . . . . . 0x180/imm32/fixed-perms # copy 0 to ECX
b8/copy . . . . . . . 8/imm32/creat # copy 8 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h

## fd = open(filename, O_WRONLY, 0)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b9/copy . . . . . . . 1/imm32/wronly # copy 1 to ECX
ba/copy . . . . . . . 0x180/imm32/fixed-perms # copy 0 to EDX
b8/copy . . . . . . . 5/imm32/open # copy 5 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
# save fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
89/copy 0/mod/indirect 3/rm32/EBX 0/r32/EAX # copy EAX to *EBX

## write(fd, "a", 1)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b9/copy . . . . . . . 0x08049129/imm32/a # copy to ECX
ba/copy . . . . . . . 1/imm32/size # copy 1 to EDX
b8/copy . . . . . . . 4/imm32/write # copy 4 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h

## close(fd)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b8/copy . . . . . . . 6/imm32/close # copy 6 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h

## fd = open(filename, O_RDONLY, 0)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b9/copy . . . . . . . 0/imm32/rdonly # copy 0 to ECX
ba/copy . . . . . . . 0x180/imm32/fixed-perms # copy 0 to EDX
b8/copy . . . . . . . 5/imm32/open # copy 5 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h
# save fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
89/copy 0/mod/indirect 3/rm32/EBX 0/r32/EAX # copy EAX to *EBX

## read(fd, b, 1)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b9/copy . . . . . . . 0x0804912d/imm32/b # copy to ECX
ba/copy . . . . . . . 1/imm32/size # copy 1 to EDX
b8/copy . . . . . . . 3/imm32/read # copy 3 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h

## close(fd)
# load fd
bb/copy . . . . . . . 0x08049125/imm32/fd # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b8/copy . . . . . . . 6/imm32/close # copy 8 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h

## unlink(filename)
bb/copy . . . . . . . 0x08049131/imm32/fname # copy to EBX
b8/copy . . . . . . . 0xa/imm32/unlink # copy 8 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h

## exit(b)
# load b
bb . . . . . . . 0x0804912d/imm32/b # copy to EBX
8b/copy 0/mod/indirect 3/rm32/EBX 3/r32/EBX # copy *EBX to EBX
#
b8/copy . . . . . . . 1/imm32/exit # copy 1 to EAX
cd/syscall . . . . . . . 0x80/imm8 # int 80h

== 0x08049125 # data segment
00 00 00 00 # fd
61 00 00 00 # a: string to write to file: 'a'
00 00 00 00 # b: space for string read from file
2e 66 6f 6f 00 00 00 00 # filename: '.foo'

# vim:ft=subx:nowrap:tw&

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