Dear Coders,
Here is a fun puzzle to kick off the new year...
Background:
when libexplain goes to print string arguments, it takes care to avoid
segfaults.
Strings are relatively easy to check for valid-ness, and libexplain uses
lstat(s, &st) < 0 && errno == EFAULT
to test if a string pointer is valid. Obviously, if (the string is not
a path && the string is longer than PATH_MAX) it's not useful, but that
usually does not happen. This is certainly easier than trapping
SIGSEGV, doing a strlen, and then restoring SIGSEGV.
The Puzzle:
Is there a simple way (preferably a single system call) to discover if a
user space memory range (rather than a C string) is all valid? E.g.
write(fildes, data, data_size) trying to discover whether or not
(data,data_size) describes a piece of user memory none of which will
segfault when accessed?
Portable posix tests are better than linux-specific tests.
Using a file descriptor may not be possible (library client could have
run out), so read(2) is tempting but no banana.
--
Regards
Peter Miller <pmil...@opensource.org.au>
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