Thank you. The problem with my approach is now clear -- and in the timing scenario you laid out, the script will have no problem writing to that file handle after the second symlink check passes. Once the "open" has succeeded, FH is pointing directly to the actual target file, and deletion of the symlink has no impact on the ability to write successfully to the file.

Well then, here's one last try: 

unless ( -l "foo" ) { # we get here fine
    #*now* the link is created
    open( FH, ">>foo" ) or die "foo: $!";
    #and now it's removed again
}
die "Link attack detected" if ( -l "foo" or ! -e _ );

# we have checked for both existence and "type == symlink" on the same
+ stat call,
# so either it's a link, or it's non-existent, or it's safe to write o
+utput

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In reply to Re^4: opening files: link checking and race conditions by graff
in thread opening files: link checking and race conditions by danderson

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