[cfe-dev] [RFC] Adding warnings against usage of %n

Jayson Yan via cfe-dev cfe-dev at lists.llvm.org
Wed Sep 22 14:32:52 PDT 2021

Hello everyone,

We’re interested in warning against the usage of the “%n” format string
specifier to discourage developers from using this potentially unsafe
format specifier.

We were thinking of enabling this as a warning under the -Wformat-security
flag but are open to alternatives, some ideas were:


   Placing this check under its own flag (something along the lines of
   “-Wformat-n”) and possibly enabling it with -Wformat or -Wformat-security


   Adding a check to clang-tidy

   Setting the default configuration based on the target triple (target
   runtimes may disallow %n altogether and we could detect this based on the
   target triple)

Interested in any thoughts on:


   How should we enable this? (eg. flags, clang-tidy, etc)

   Should we surface this as an error or a warning?

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