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Problem accessing /proc/<pid>/mem range after mprotect has made it readable

Posted by voidlogic 
Hey Everyone,

I am having a problem accessing /proc/<pid>/mem range after mprotect has made it readable.

    1. Process A ptrace ATTACHs to process B
    2. Process A cannot access of a given range in process Bs /proc/<pid>/mem due to the associated mapping (as visible in /proc/<pid>/maps) being non-readable
    3. Process A requests process B use mprotect to mark given mapping as readable (mprotect returns success)
    4. Process A now should be able to read the range (as visible in /proc/<pid>/mem) but can only read part of the range...

Mapping Before Request (Step 3):
7f63671af000-7f63673ae000 ---p 0000c000 08:01 269062     /lib/x86_64-linux-gnu/libnss_files-2.13.so
Mapping After Request (Step 3):
7f63671af000-7f63673ae000 r--p 0000c000 08:01 269062     /lib/x86_64-linux-gnu/libnss_files-2.13.so

Problem: std::ifstream can read a first part of the range from /proc/<pid>/mem, but cannot read the remaining range data. (By cannot read I mean !fin.good and errno = 5)

Mapping Range:             7f63671af000-7f63673ae000 (2044 Kbytes)
Successfully Read Range:   7f63671af000-7f63671affff (4095 bytes, ~one page)
Unsuccessfully Read Range: 7f63671b0000-7f63673ae000 (2040 Kbytes)

I do not believe the error is related to procfs because if rather than reading /proc/<pid>/mem, I attempt to access the data word by word using PTRACE_PEEKTEXT, the result is still failure on the same boundaries.

I have also tried calling mprotect again on just the trouble region with no change.

Any ideas or hints on what I might be doing wrong would be great! Or- is it a possibly a bug? Let me know if there is any further information I can provide.

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