20449bcc9f
Co-authored-by: Tarun Prabhu <tarunprabhu@gmail.com> Co-authored-by: David Spickett <david.spickett@linaro.org>
53 lines
2.5 KiB
Markdown
53 lines
2.5 KiB
Markdown
<!--===- docs/FAQ.md
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Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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See https://llvm.org/LICENSE.txt for license information.
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SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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-->
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# Frequently Asked Questions (FAQ)
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```{contents}
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---
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local:
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---
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```
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## Driver
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### Why do I get a warning or an error about an executable stack?
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This occurs because Flang's implementation of pointers to internal procedures requires an executable stack.
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An internal procedure has a "host scope", which is the scope in which it is contained.
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It can access variables defined in that host scope.
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When an internal procedure is referenced from outside its host scope (for example, via a procedure pointer), the implementation must ensure that it can still access variables from the host scope.
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To achieve this, the current implementation of Flang generates a small piece of code, called a "trampoline", on the stack.
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When the procedure is called, this trampoline is executed.
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The trampoline is on the stack, so the stack itself must be executable.
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For a more detailed explanation of trampolines, please refer to the [design document](InternalProcedureTrampolines.md).
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An executable stack increases the risk and impact of certain classes of security vulnerabilities, such as [stack buffer overflows](https://llsoftsec.github.io/llsoftsecbook/#stack-buffer-overflows).
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Therefore, modern linkers often issue a warning or an error if an executable stack is not explicitly requested by the developer.
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For instance, the GNU Linker (`ld`) issues a warning while the LLVM Linker (`lld`) emits an error.
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```{note}
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The trampoline code generated by Flang is not itself a security risk.
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The risk comes from the possibility of executing malicious code that an attacker has placed on the stack.
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You should determine whether such risks are appropriate for your software.
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```
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When you use the Flang driver (the `flang` command) to generate executables, you can instruct the linker to enable an executable stack with the `-Wl,-z,execstack` or `-Xlinker -zexecstack` flag.
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```console
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$ flang src.f90 -fuse-ld=ld
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/path/to/ld: warning: src.o: requires executable stack (because the .note.GNU-stack section is executable)
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$ flang src.f90 -fuse-ld=ld -Wl,-z,execstack
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$ flang src.f90 -fuse-ld=lld
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ld.lld: error: src.o: requires an executable stack, but -z execstack is not specified
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flang-22: error: linker command failed with exit code 1 (use -v to see invocation)
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$ flang src.f90 -fuse-ld=lld -Wl,-z,execstack
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```
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