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Old 09-30-2012, 02:10 PM   #1
kronus
LQ Newbie
 
Registered: Sep 2012
Posts: 4

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Few Memory location within a page not accessible from GDB


Hi,

I am seeing this issue, where few memory location within a page are not accessible from GDB, but others are.

Now my doubt is, if the page entry in page table doesn't have PTE_U set, then whole page should not be accessible.

Could anyone give me some idea, why this could happen, so that I can fix the issue.

(gdb) p /x *( 0xef7fdfff-5)
$12 = 0x0
(gdb) p /x *( 0xef7fdfff-4)
$13 = 0x0
(gdb) p /x *( 0xef7fdfff-3)
$14 = 0x0
(gdb) p /x *( 0xef7fdfff-2)
Cannot access memory at address 0xef7fdffd
(gdb) p /x *( 0xef7fdfff-1)
Cannot access memory at address 0xef7fdffe
(gdb) p /x *( 0xef7fdfff)
Cannot access memory at address 0xef7fdfff

Thanks
 
Old 10-01-2012, 08:26 AM   #2
johnsfine
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Registered: Dec 2007
Distribution: Centos
Posts: 5,286

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Quote:
Originally Posted by kronus View Post
I am seeing this issue, where few memory location within a page are not accessible from GDB, but others are.
I am not sure which of two things you are misunderstanding:

1) There is a default size for the memory reference *(address) in gdb. I couldn't found that described in gdb documentation (probably is there and I don't read such documentation well). But a google search found a "hacking" site that at least told me that default size in 32 bit Linux is 32 bits.

2) Each 8 bits has its own address. A 32 bit memory reference is effectively accessing four sequential addresses starting at the one you specify. All four need to be mapped, not just the first one.
 
  


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