kernel system call handlers missing due to -Wl,--no-whole-archive #5184
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area: Memory Protection
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andrewboie
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area: Memory Protection
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@andrewboie , can you please set the priority? |
lpereira
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priority: high
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I had originally set this to high because this wouldn't work on ARM, but userland isn't working on ARM yet and won't be at least for 1.11. So moved back to medium. |
andrewboie
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Nov 29, 2017
The core kernel is built with the --no-whole-archive linker option. For all the individual .o files which make up the kernel, if there are no external references to symbols within these object files, everything in the object file is dropped. This has a subtle interaction with system call handlers. If an object file has system call handlers inside it, and nothing else in the object file is referenced, then the linker will prefer the weak version of the handler in the generated syscall_dispatch.c. The user will get an "unimplemented system call" error if the associated system call for that handler is made. Fix this by making a fake reference to the handler function at the system call site. The address gets stored inside a special section "hndlr_ref". This is enough to prevent the handlers from being dropped, and the hndlr_ref section is itself dropped from the binary from gc-sections; these references will not consume space. Handlers for system calls that are never invoked anywhere will still be dropped if nothing else in their containing C files is used, which is a good thing. A future enhancement could be to split out all handlers into individual object files, such that we can guarantee that any system call that is not made somewhere in the application will have its handler dropped. This will need to be extended to driver subsystems as well. This won't be pretty but will ensure the tightest binary size. Fixes zephyrproject-rtos#5184. Signed-off-by: Andrew Boie <[email protected]>
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This issue affects x86 and needs to go into 1.10 |
andrewboie
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andrewboie
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Nov 29, 2017
The core kernel is built with the --no-whole-archive linker option. For all the individual .o files which make up the kernel, if there are no external references to symbols within these object files, everything in the object file is dropped. This has a subtle interaction with system call handlers. If an object file has system call handlers inside it, and nothing else in the object file is referenced, then the linker will prefer the weak version of the handler in the generated syscall_dispatch.c. The user will get an "unimplemented system call" error if the associated system call for that handler is made. Fix this by making a fake reference to the handler function at the system call site. The address gets stored inside a special section "hndlr_ref". This is enough to prevent the handlers from being dropped, and the hndlr_ref section is itself dropped from the binary from gc-sections; these references will not consume space. Handlers for system calls that are never invoked anywhere will still be dropped if nothing else in their containing C files is used, which is a good thing. A future enhancement could be to split out all handlers into individual object files, such that we can guarantee that any system call that is not made somewhere in the application will have its handler dropped. This will need to be extended to driver subsystems as well. This won't be pretty but will ensure the tightest binary size. Fixes #5184. Signed-off-by: Andrew Boie <[email protected]>
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@AdithyaBaglody discovered that this problem is still happening, my patch didn't completely fix it. |
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@AdithyaBaglody found a fix |
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Discovered by @agross-linaro, who was getting "unimplemented system call" errors on ARM for k_thread_abort().
ARM has a custom implementation of k_thread_abort under arch/arm. This has the effect of compiling out everything in kernel/thread_abort.c except the handler function.
For some strange reason, if --no-whole-archive is enabled, the linker decides to prefer the weak handler for k_thread_abort() in syscall_dispatch.c.
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