On Wed, Feb 19, 2020 at 3:34 AM Mark Rutland [off-list ref] wrote:

Fangrui Song has implemented `-fpatchable-function-entry` in LLVM (for
10.x onwards), so we can support this when DYNAMIC_FTRACE_WITH_REGS is
selected.

This can be:

        depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER

... and we can update the commit message to something like:

| With SCS the return address is taken from the shadow stack and the
| value in the frame record has no effect. The mcount based graph tracer
| hooks returns by modifying frame records on the (regular) stack, and
| thus is not compatible. The patchable-function-entry graph tracer
| used for DYNAMIC_FTRACE_WITH_REGS modifies the LR before it is saved
| to the shadow stack, and is compatible.
|
| Modifying the mcount based graph tracer to work with SCS would require
| a mechanism to determine the corresponding slot on the shadow stack
| (and to pass this through the ftrace infrastructure), and we expect
| that everyone will eventually move to the patchable-function-entry
| based graph tracer anyway, so for now let's disable SCS when the
| mcount-based graph tracer is enabled.
|
| SCS and patchable-function-entry are both supported from LLVM 10.x.

Assuming you're happy with that:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Great, thanks for pointing that out! This looks good to me, I'll use this in v9.

Sami

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