On 10/15/20 1:59 PM, Nick Desaulniers wrote:

On Thu, Oct 8, 2020 at 1:30 AM Russell King - ARM Linux admin
[off-list ref] wrote:

quoted

On Thu, Oct 08, 2020 at 12:45:30PM +0530, Maninder Singh wrote:

quoted

Observed Stack Overflow on 8KB kernel stack on ARM specially
incase on network interrupts, which results in undeterministic behaviour.
So there is need for per cpu dedicated IRQ stack for ARM.

As ARm does not have extra co-processor register
to save thread info pointer, IRQ stack will be at some
performance cost, so code is under CONFIG_IRQ_STACK.

and we don't have much knowledge and set up for CLANG
and ARM_UNWIND, so dependency added for both cases.

Tested patch set with QEMU for latest kernel
and 4.1 kernel for ARM target with same patch set.

You need to investigate and show where and why this is happening. My
guess is you have a network driver that uses a lot of kernel stack
space, which itself would be a bug.

Note that there are compiler versions out there that mis-optimise and
eat stack space - the kernel build should be warning if a function
uses a large amount of stack.

For tracking down those not-super-helpful compiler warnings, I wrote a
tool where if you rebuild with debug info, and give it the object file
and string of the function the compiler warned about it will parse the
DWARF to tell you the size of each local variable, and if it came from
an inline frame.  Generally, it's possible to stack allocate something
that's way too big; instead those should be allocated on the heap.
https://github.com/ClangBuiltLinux/frame-larger-than
(I haven't had time to sit down and use it to resolve all outstanding
issues, but it has worked well for me in the past)

Things get a bit more difficult with the network stack and you easily
recurse into functions and blow up the stack size. This is especially
true if you have some complex network tunneling or filtering going on.

For one, in the 4.1 kernel that appears to have been used as a basis for
this work, if you have CONFIG_BPF enabled but not
CONFIG_BPF_JIT_ALWAYS_ON, __bpf_prog_run will require about 724 bytes of
stack last I measured, that's nearly 10% of the stack that goes away
just like that.
--
Florian

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