Regression with -D_FORTIFY_SOURCE=2

[tstellar]

There was a regression compiling libmbim between clang-14 and clang-15, and it's still present in main.

The bad behavior began after bfb9b8e, however, I think the root cause is a bug in Clang's handling of -D_FORTIFY_SOURCE=2. When building libmbim, the libmbim-glib-scan utility hangs, and from some limited debugging I did, it appears that it gets stuck in a loop of strncpy calling itself, which I think has happened before due to inlining of the *_chk functions in glibc.

I am able to reliably reproduce this bug in a Fedora 37 container using the script below:

sudo dnf install 'dnf-command(builddep)' git
sudo dnf builddep libmbim
git clone https://github.com/freedesktop/libmbim
cd libmbim
git checkout 1.26.4

export CFLAGS='-O2 -flto -g -Wp,-D_FORTIFY_SOURCE=2 -Wno-cast-align -gdwarf-4'
export LDFLAGS='-flto -fuse-ld=lld'
export CC=clang
export AR=llvm-ar
export RANLIB=llvm-ranlib
export NM=llvm-nm

./autogen.sh

./configure --build=x86_64-redhat-linux-gnu --host=x86_64-redhat-linux-gnu --program-prefix= --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64 --libexecdir=/usr/libexec --localstatedir=/var --sharedstatedir=/var/lib --mandir=/usr/share/man --infodir=/usr/share/info --disable-static --enable-gtk-doc

/usr/bin/make -O -j8 V=1 VERBOSE=1

@serge-sans-paille

[efriedma-quic]

Clang has to do some weird stuff to "correctly" lower the strange functions that call themselves defined by glibc _FORTIFY_SOURCE. I think this was last touched by https://reviews.llvm.org/D134362 ? We constantly run into edge cases involving this kind of code.

Could use a reduced testcase.

[tstellar]

Here is a reduced testcase

Compile with

clang -flto -x c -std=gnu89 -O2 -S -emit-llvm reduced.ii -o reduced.ll

The generated strncpy function looks wrong:

; Function Attrs: alwaysinline nofree nounwind willreturn memory(argmem: readwrite) uwtable
define available_externally ptr @strncpy(ptr noalias noundef nonnull returned writeonly %0, ptr noalias nocapture noundef nonnull readonly %1, i64 noundef %2) local_unnamed_addr #3 {
  br label %4

4:                                                ; preds = %4, %3
  %5 = phi ptr [ poison, %3 ], [ %7, %4 ]
  %6 = phi i1 [ false, %3 ], [ true, %4 ]
  %7 = select i1 %6, ptr %5, ptr %0
  br label %4
}

[serge-sans-paille]

https://reviews.llvm.org/D147307 << potential fix.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[nikic]

/cherry-pick fd8745c

[llvmbot]

/branch llvm/llvm-project-release-prs/issue61691

[llvmbot]

/pull-request llvm/llvm-project-release-prs#417

[tstellar]

Since this commit introduced a regression, we will not be backporting it to 16.0.x.

[nikic]

Closing this as the issue has been fixed and this was now tracking the backport -- with the backport declined, we can close this again.