path: root/linux/313index-bounds-check.subx

# Helper to check an array's bounds, and to abort if they're violated.
# Really only intended to be called from code generated by mu.subx.

== code

__check-mu-array-bounds:  # index: int, elem-size: int, arr-size: int, function-name: (addr array byte), array-name: (addr array byte)
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    50/push-eax
    51/push-ecx
    52/push-edx
    # . not bothering saving ebx; it's only clobbered if we're going to abort
    # ecx = arr-size
    8b/-> *(ebp+0x10) 1/r32/ecx
    # var overflow/edx: int = 0
    ba/copy-to-edx 0/imm32
    # var offset/eax: int = index * elem-size
    8b/-> *(ebp+8) 0/r32/eax
    f7 4/subop/multiply-eax-with *(ebp+0xc)
    # check for overflow
    81 7/subop/compare %edx 0/imm32
    0f 85/jump-if-!= __check-mu-array-bounds:overflow/disp32
    # check bounds
    39/compare %eax 1/r32/ecx
    0f 82/jump-if-unsigned< $__check-mu-array-bounds:end/disp32  # negative index should always abort
    # abort if necessary
    (write-buffered Stderr "fn ")
    (write-buffered Stderr *(ebp+0x14))
    (write-buffered Stderr ": offset ")
    (write-int32-hex-buffered Stderr %eax)
    (write-buffered Stderr " is too large for array '")
    (write-buffered Stderr *(ebp+0x18))
    (write-buffered Stderr "'\n")
    (flush Stderr)
    # exit(1)
    bb/copy-to-ebx 1/imm32
    e8/call syscall_exit/disp32
    # never gets here
$__check-mu-array-bounds:end:
    # . restore registers
    5a/pop-to-edx
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

__check-mu-array-bounds:overflow:
    # "fn " function-name ": offset to array '" array-name "' overflowed 32 bits\n"
    (write-buffered Stderr "fn ")
    (write-buffered Stderr *(ebp+0x14))
    (write-buffered Stderr ": offset to array '")
    (write-buffered Stderr *(ebp+0x18))
    (write-buffered Stderr "' overflowed 32 bits\n")
    (flush Stderr)
    # exit(1)
    bb/copy-to-ebx 1/imm32
    e8/call syscall_exit/disp32
    # never gets here

# potential alternative

#? __bounds-check:  # msg: (addr array byte)
#?   (write-buffered Stderr "abort: array bounds exceeded in fn ")
#?   8b/-> *(esp+4) 0/r32/eax  # we're going to abort, so just clobber away
#?   (write-buffered Stderr %eax)
#?   (write-buffered Stderr Newline)
#?   # exit(1)
#?   bb/copy-to-ebx 1/imm32
#?   e8/call syscall_exit/disp32

# to be called as follows:
#   var/reg <- index arr/rega: (addr array T), idx/regi: int
#     | if size-of(T) is 1, 2, 4 or 8
#         => # temporarily save array size to reg to check bounds
#            "8b/-> *" rega " " reg "/r32"
#            "c1/shift 5/subop/right %" reg " " log2(size-of(T)) "/imm32"
#            "3b/compare " reg "/r32 *" rega
#            "68/push \"" function "\"/imm32"  # pass function name to error message
#            "0f 8d/jump-if->= __bounds_check/disp32"
#            "81 0/subop/add %esp 4/imm32"  # drop function name
#            # actually save the index addr in reg
#            "8d/copy-address *(" rega "+" regi "<<" log2(size-of(T)) "+4) " reg "/r32"