2.3 KiB
This page explains the following warning message and how to fix it. mold emits this message when it sees an object file that may not be compatible with mold.
mold: warning: foo.o: this file may cause a segmentation fault because it requires an executable stack. See https://github.com/rui314/mold/tree/main/docs/execstack.md for more info.
Background
On modern computers, the stack area (to which local variables are stored) cannot contain executable code. If the control reaches the stack area, the CPU refuses to execute any code there and the program is usually terminated due to segmentation fault.
This is a security measure. The stack area used to be executable (old CPUs generally execute any code as long as it is in a readable memory region), but that provided an easy attack vector to a malicious user. They wrote executable code to the stack area using some buffer overflow bug and jumped there to run arbitrary code in a remote server process.
To prevent this type of attack, the stack area is no longer executable since the early 2000s. On Linux, the stack's executable-ness is controlled by a bit in an executable, and the loader respects that bit. The bit is set by the linker.
GCC had (and still has) a feature that depends on the executable
stack, so they invented a way to tell the linker to mark the stack
executable. Specifically, if an object file contains a
.note.GNU-stack section with the SHF_EXECSTACK bit, GNU linker
silently makes the stack of an output file executable.
But the GNU linker's behavior is dangerous. If you accidentally link an object file that has that marker section, the entire stack area silently becomes executable, disabling the security mechanism.
Therefore, mold simply ignores that marker section. If you are using
mold, you need to explicitly pass -z execstack to the linker to make
the stack executable.
What caused this issue?
You are likely to use GCC's Nested Functions feature which still depends on the executable stack.
How to fix it?
If you know what you are doing, pass -z execstack to mold. Beware
that this will significantly weaken your program's security.
If you don't want to pass -z execstack, rewrite your code so that
your code does not depend on the executable stack.