kernel_5.10 no rt

hc

2023-12-11 6778948f9de86c3cfaf36725a7c87dcff9ba247f

 kernel/arch/arm64/kernel/vdso/sigreturn.S
..
..
@@ -1,37 +1,80 @@
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+/* SPDX-License-Identifier: GPL-2.0-only */
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 /*
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  * Sigreturn trampoline for returning from a signal when the SA_RESTORER
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- * flag is not set.
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+ * flag is not set. It serves primarily as a hall of shame for crappy
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+ * unwinders and features an exciting but mysterious NOP instruction.
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+ *
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+ * It's also fragile as hell, so please think twice before changing anything
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+ * in here.
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  *
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  * Copyright (C) 2012 ARM Limited
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- *
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- * This program is free software; you can redistribute it and/or modify
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- * it under the terms of the GNU General Public License version 2 as
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- * published by the Free Software Foundation.
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- *
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- * This program is distributed in the hope that it will be useful,
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- * but WITHOUT ANY WARRANTY; without even the implied warranty of
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- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
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- * GNU General Public License for more details.
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- *
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- * You should have received a copy of the GNU General Public License
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- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
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  *
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  * Author: Will Deacon <will.deacon@arm.com>
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  */
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14
 
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 #include <linux/linkage.h>
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+#include <asm/assembler.h>
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 #include <asm/unistd.h>
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 	.text
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-	nop
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-ENTRY(__kernel_rt_sigreturn)
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-	.cfi_startproc
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-	.cfi_signal_frame
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-	.cfi_def_cfa	x29, 0
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-	.cfi_offset	x29, 0 * 8
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-	.cfi_offset	x30, 1 * 8
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+/*
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+ * NOTE!!!  You may notice that all of the .cfi directives in this file have
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+ * been commented out. This is because they have been shown to trigger segfaults
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+ * in libgcc when unwinding out of a SIGCANCEL handler to invoke pthread
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+ * cleanup handlers during the thread cancellation dance. By omitting the
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+ * directives, we trigger an arm64-specific fallback path in the unwinder which
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+ * recognises the signal frame and restores many of the registers directly from
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+ * the sigcontext. Re-enabling the cfi directives here therefore needs to be
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+ * much more comprehensive to reduce the risk of further regressions.
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+ */
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+
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+/* Ensure that the mysterious NOP can be associated with a function. */
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+//	.cfi_startproc
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+
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+/*
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+ * .cfi_signal_frame causes the corresponding Frame Description Entry (FDE) in
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+ * the .eh_frame section to be annotated as a signal frame. This allows DWARF
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+ * unwinders (e.g. libstdc++) to implement _Unwind_GetIPInfo() and identify
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+ * the next frame using the unmodified return address instead of subtracting 1,
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+ * which may yield the wrong FDE.
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+ */
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+//	.cfi_signal_frame
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+
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+/*
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+ * Tell the unwinder where to locate the frame record linking back to the
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+ * interrupted context. We don't provide unwind info for registers other than
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+ * the frame pointer and the link register here; in practice, this is likely to
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+ * be insufficient for unwinding in C/C++ based runtimes, especially without a
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+ * means to restore the stack pointer. Thankfully, unwinders and debuggers
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+ * already have baked-in strategies for attempting to unwind out of signals.
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+ */
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+//	.cfi_def_cfa    x29, 0
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+//	.cfi_offset     x29, 0 * 8
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+//	.cfi_offset     x30, 1 * 8
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+
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+/*
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+ * This mysterious NOP is required for some unwinders (e.g. libc++) that
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+ * unconditionally subtract one from the result of _Unwind_GetIP() in order to
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+ * identify the calling function.
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+ * Hack borrowed from arch/powerpc/kernel/vdso64/sigtramp.S.
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+ */
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+	nop	// Mysterious NOP
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+
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+/*
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+ * GDB, libgcc and libunwind rely on being able to identify the sigreturn
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+ * instruction sequence to unwind from signal handlers. We cannot, therefore,
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+ * use SYM_FUNC_START() here, as it will emit a BTI C instruction and break the
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+ * unwinder. Thankfully, this function is only ever called from a RET and so
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+ * omitting the landing pad is perfectly fine.
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+ */
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+SYM_CODE_START(__kernel_rt_sigreturn)
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+//	PLEASE DO NOT MODIFY
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 	mov	x8, #__NR_rt_sigreturn
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+//	PLEASE DO NOT MODIFY
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 	svc	#0
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-	.cfi_endproc
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-ENDPROC(__kernel_rt_sigreturn)
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+//	PLEASE DO NOT MODIFY
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+//	.cfi_endproc
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+SYM_CODE_END(__kernel_rt_sigreturn)
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+
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+emit_aarch64_feature_1_and