linux-os (Dick Johnson) wrote:
On Fri, 12 May 2006, Tomasz Malesinski wrote:
The code attached below segfaults on the enter instruction. It works
when a stack frame is created by the three commented out
instructions and also when the first operand of the enter instruction
is small (less than about 6500 on my system).
AFAIK, the only difference between creating a stack frame with the
enter instruction or push/mov/sub is that enter checks if the new
value of esp is inside the stack segment limit.
I tested it on a vanilla kernel 2.4.26 on Intel Celeron and also on
probably non-vanilla 2.6.16.13 running on 3 dual core AMD Opteron,
quite busy, server. It is working in 32-bit mode. Interestingly, on
the second machine sometimes the program worked correctly.
I am not subscribed to the list. Please cc replies to me.
.file "a.c"
.version "01.01"
gcc2_compiled.:
.section .rodata
.LC0:
.string "asdf\n"
.text
.align 4
.globl main
.type main,@function
main:
enter $10008, $0
# pushl %ebp
# movl %esp,%ebp
# subl $10008,%esp
addl $-12,%esp
^^^^^^^^^^^^^^____________ WTF
adding a negative number is subtracting that positive value.
Right, adding -12 is the same as subtracting 12. I have no idea what
you're getting at with the next two lines.
You just subtracted 0xfffffff3 (on a 32-bit machine) from
the stack pointer. It damn-well better seg-fault!
No, we subtracted 12. I'm not sure where that number came from, it's the
1's complement of 12 but I'm dead sure Linux code isn't running on any
1's comp machines.
--
-bill davidsen ([email protected])
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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