[Bug gdb/17846] New: read memory from _dl_debug_state returns 0xcc instead of 0xf3

["ganzmatthias at hotmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17846

            Bug ID: 17846
           Summary: read memory from _dl_debug_state returns 0xcc instead
                    of 0xf3
           Product: gdb
           Version: 7.8
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: ganzmatthias at hotmail dot com

Created attachment 8065
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8065&action=edit
Small example to reproduce the issue with some information about my environment

When trying to read one byte from _dl_debug_state (I try to read and analyse
arbitrary code loaded into my memory) the behavior of my program is different
depending on whether I run it with gdb or not.

Memory at symbol _dl_debug_state (which is at a fixed address 0x7ffff7dea970
for my system) starts with the values 0xf3 c3. Which corresponds to a "repz
ret" instruction. When i try to read the very first byte of this symbol I read
0xCC (which corresponds to a int3 instruction) instead of 0xF3.

However when I inspect the memory address with gdb as following, I get the
correct value 0xf3.

(gdb) x/1bx 0x7ffff7dea970
0x7ffff7dea970 <_dl_debug_state>:    0xf3

But my program accidentally seems to read the trap instruction instead of the
actual memory value.

The attached archive contains a small example, the compiled binary and a log
how to reproduce the issue.

If I run the binary on its own everything is fine but if it is run within gdb I
get the previously described error.

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