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This patch enables `-fpatchable-function-entry` on PPC64 little-endian
Linux. It is mutually exclusive with existing XRay instrumentation on
this target.
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With -fpatchable-function-entry (or the patchable_function_entry
function attribute), we emit records of patchable entry locations to the
__patchable_function_entries section. Add an additional parameter to the
command line option that allows one to specify a different default
section name for the records, and an identical parameter to the function
attribute that allows one to override the section used.
The main use case for this change is the Linux kernel using prefix NOPs
for ftrace, and thus depending on__patchable_function_entries to locate
traceable functions. Functions that are not traceable currently disable
entry NOPs using the function attribute, but this creates a
compatibility issue with -fsanitize=kcfi, which expects all indirectly
callable functions to have a type hash prefix at the same offset from
the function entry.
Adding a section parameter would allow the kernel to distinguish between
traceable and non-traceable functions by adding entry records to
separate sections while maintaining a stable function prefix layout for
all functions. LKML discussion:
https://lore.kernel.org/lkml/Y1QEzk%2FA41PKLEPe@hirez.programming.kicks-ass.net/
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For now only PPC big endian Linux 32 and 64 bit are supported.
PPC little endian Linux has XRAY support for 64-bit.
PPC AIX has different patchable function entry implementations.
Fixes #63220
Fixes #57031
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Similar to previous cleanup.
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Similar to D98610 for RISCV.
This is going to be required by the upcoming Linux/LoongArch
[[ https://git.kernel.org/linus/4733f09d88074 | support for dynamic ftrace ]].
Reviewed By: SixWeining, MaskRay
Differential Revision: https://reviews.llvm.org/D141785
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'patchable_function_entry'
Similar to D72215 (AArch64) and D72220 (x86).
```
% clang -target riscv32 -march=rv64g -c -fpatchable-function-entry=2 a.c && llvm-objdump -dr a.o
...
0000000000000000 <main>:
0: 13 00 00 00 nop
4: 13 00 00 00 nop
% clang -target riscv32 -march=rv64gc -c -fpatchable-function-entry=2 a.c && llvm-objdump -dr a.o
...
00000002 <main>:
2: 01 00 nop
4: 01 00 nop
```
Recently the mainline kernel started to use -fpatchable-function-entry=8 for riscv (https://git.kernel.org/linus/afc76b8b80112189b6f11e67e19cf58301944814).
Differential Revision: https://reviews.llvm.org/D98610
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__attribute__((patchable_function_entry(N,M))) where M>0
Reviewed By: nickdesaulniers
Differential Revision: https://reviews.llvm.org/D73072
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In the backend, this feature is implemented with the function attribute
"patchable-function-entry". Both the attribute and XRay use
TargetOpcode::PATCHABLE_FUNCTION_ENTER, so the two features are
incompatible.
Reviewed By: ostannard, MaskRay
Differential Revision: https://reviews.llvm.org/D72222
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