URL:
<http://savannah.gnu.org/bugs/?43222>
Summary: memcpy is not a safe fallback for bcopy/memmove
Project: GNU Screen
Submitted by: maartenth
Submitted on: Mon 15 Sep 2014 02:03:54 AM GMT
Category: Program Logic
Severity: 3 - Normal
Priority: 5 - Normal
Status: None
Privacy: Public
Assigned to: None
Open/Closed: Open
Discussion Lock: Any
Release: 4.2.1
Fixed Release: None
Planned Release: None
Work Required: None
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Details:
Screen tries 4 alternatives for handing memory copies that potentially
overlap, in this order:
- bcopy from libc
- memmove
- memcpy
- bcopy from Screen (xbcopy)
However, memcpy is not a safe alternative, since its behavior when the memory
regions overlap is explicitly left undefined by the specification. The
configure script tries to work around that by testing the behavior of memcpy
on overlapping memory, but if the test passes that doesn't provide any
guarantee that other overlapping regions will also be copied correctly. This
is not just a theoretical concern: memcpy implementations can have different
code paths depending on pointer alignment and size of the block to be copied
and this test would only excercise a single code path.
In practice systems with broken bcopy/memmove will be rare these days, so most
systems will not be affected. But for those where a fallback is necessary, it
is best to fall back on Screen's own bcopy, which might be slower but is at
least reliable.
I attached a patch that removes memcpy as a fallback.
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File Attachments:
-------------------------------------------------------
Date: Mon 15 Sep 2014 02:03:54 AM GMT Name:
0001-Do-not-use-memcpy-as-an-alternative-for-bcopy-memmov.patch Size: 3kB
By: maartenth
git formatted patch
<http://savannah.gnu.org/bugs/download.php?file_id=32096>
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