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Don't follow symlinks on file creation (CVE-2017-7501)
Open newly created files with O_EXCL to prevent symlink tricks. When reopening hardlinks for writing the actual content, use append mode instead. This is compatible with the write-only permissions but is not destructive in case we got redirected to somebody elses file, verify the target before actually writing anything. As these are files with the temporary suffix, errors mean a local user with sufficient privileges to break the installation of the package anyway is trying to goof us on purpose, don't bother trying to mend it (we couldn't fix the hardlink case anyhow) but just bail out. Based on a patch by Florian Festi.
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