linux-kernel - [PATCH v4 31/31] ARM: entry: Straighten syscall returns

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Message-Id: <20250212-arm-generic-entry-v4-31-a457ff0a61d6@linaro.org>
Date: Wed, 12 Feb 2025 12:23:25 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Dmitry Vyukov <dvyukov@...gle.com>, Oleg Nesterov <oleg@...hat.com>, 
 Russell King <linux@...linux.org.uk>, Kees Cook <kees@...nel.org>, 
 Andy Lutomirski <luto@...capital.net>, Will Drewry <wad@...omium.org>, 
 Frederic Weisbecker <frederic@...nel.org>, 
 "Paul E. McKenney" <paulmck@...nel.org>, 
 Jinjie Ruan <ruanjinjie@...wei.com>, Arnd Bergmann <arnd@...db.de>, 
 Ard Biesheuvel <ardb@...nel.org>, Al Viro <viro@...iv.linux.org.uk>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH v4 31/31] ARM: entry: Straighten syscall returns

Since ret_fast_syscall() is now only called in two spots in
vector_swi, we can inline the function into vector_swi, and
skip the separate call, placing the code from ret_fast_syscall
at the end of vector_swi and one of them inside an ifdef that
gets compiled out for !EABI.

TODO: is the UNWIND(.fnstart/.cantunwind/.fnend) really needed
on vector_swi after this change or can I just drop it?
I do not understand unwind well enough :/

Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
---
 arch/arm/kernel/entry-common.S | 32 ++++++++++++++------------------
 1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index a5a0e757ab23d151485092a078af98966e039869..f9c38f5311851e299234997796cd78de06a5c531 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -30,21 +30,6 @@ saved_pc	.req	lr

 	.section .entry.text,"ax",%progbits
 	.align	5
-/*
- * As we will need to call out to some C functions,
- * we save r0 first to avoid needing to save registers around each C function
- * call.
- */
-ret_fast_syscall:
- UNWIND(.fnstart	)
- UNWIND(.cantunwind	)
-	add	sp, sp, #(S_R0 + S_OFF)
-#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
-	bl	stackleak_erase_on_task_stack
-#endif
-	restore_user_regs
- UNWIND(.fnend		)
-ENDPROC(ret_fast_syscall)

 /*
  * "slow" syscall return path.  "why" tells us if this was a real syscall.
@@ -122,6 +107,8 @@ ENDPROC(vector_bhb_bpiall_swi)
 #endif
 	.align	5
 ENTRY(vector_swi)
+ UNWIND(.fnstart	)
+ UNWIND(.cantunwind	)
 #ifdef CONFIG_CPU_V7M
 	v7m_exception_entry
 #else
@@ -210,9 +197,9 @@ ENTRY(vector_swi)
 	mov	r0, tbl
 	mov	r2, scno			@ syscall number from r7
 	bl	invoke_syscall
-	b	ret_fast_syscall
-
 #if defined(CONFIG_OABI_COMPAT) || !defined(CONFIG_AEABI)
+	b	9002
+
 	/*
 	 * We failed to handle a fault trying to access the page
 	 * containing the swi instruction, but we're not really in a
@@ -225,8 +212,17 @@ ENTRY(vector_swi)
 	sub	lr, saved_pc, #4
 	str	lr, [sp, #S_PC]
 	get_thread_info tsk
-	b	ret_fast_syscall
+9002:
+	/* Exit !AEABI clause */
+#endif
+
+	/* Return from syscall */
+	add	sp, sp, #(S_R0 + S_OFF)
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	bl	stackleak_erase_on_task_stack
 #endif
+	restore_user_regs
+ UNWIND(.fnend		)
 ENDPROC(vector_swi)
 	.ltorg

-- 
2.48.1