Re: [Libguestfs] [PATCH nbdkit] Ban use of stack Variable Length Arrays (VLAs).

On 9/5/19 6:28 AM, Richard W.M. Jones wrote: > I'm not someone who thinks VLAs are automatically bad and unlike Linux > kernel code they can sometimes be used safely in userspace. However > for an internet exposed server there is an argument that they might > cause some kind of exploitable situation especially if the code is > compiled without other stack hardening features. Also in highly > multithreaded code with limited stack sizes (as nbdkit is on some > platforms) allowing unbounded stack allocation can be a bad idea > because it can cause a segfault. > > So this commit bans them. Only when using --enable-gcc-warnings, but > upstream developers ought to be using that. > > There were in fact only two places where VLAs were being used. In one > of those places (plugins/sh) removing the VLA actually made the code > better. > > For interesting discussion about VLAs in the kernel see: > https://lwn.net/Articles/763253/ > --- > configure.ac | 2 +- > plugins/sh/sh.c | 7 +++---- > server/sockets.c | 8 +++++++- > 3 files changed, 11 insertions(+), 6 deletions(-) > +++ b/configure.ac > @@ -90,7 +90,7 @@ AC_ARG_ENABLE([gcc-warnings], > [gcc_warnings=no] > ) > if test "x$gcc_warnings" = "xyes"; then > - WARNINGS_CFLAGS="-Wall -Wshadow -Werror" > + WARNINGS_CFLAGS="-Wall -Wshadow -Wvla -Werror" I'm guessing that both gcc and clang are okay with our current list; we may reach the point where we need to probe at configure time on which options we can safely use, instead of merely open-coding a list, but we'll deal with that when it breaks the build.