linux-kernel - [PATCH v3 6/8] crypto: x86/crc32c - Use idiomatic relative jump table

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Message-ID: <20241011170847.334429-16-ardb+git@google.com>
Date: Fri, 11 Oct 2024 19:08:54 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: llvm@...ts.linux.dev, keescook@...omium.org, 
	linux-hardening@...r.kernel.org, nathan@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Jan Beulich <jbeulich@...e.com>, 
	"Jose E. Marchesi" <jemarch@....org>, Kees Cook <kees@...nel.org>
Subject: [PATCH v3 6/8] crypto: x86/crc32c - Use idiomatic relative jump table

From: Ard Biesheuvel <ardb@...nel.org>

The original crc32c code used a place-relative jump table but with a
slightly awkward use of two separate symbols. To help objtool, this was
replaced with a bog-standard position dependent jump table call, which
was subsequently tweaked to use a RIP-relative reference to the table,
but still populate it with absolute 64-bit references.

Given that objtool will need to be taught about the jump table idiom
that compilers use when running with -fpie enabled, let's update the
jump table in the crc32c code once again to use this standard idiom,
where the jump table carries 32-bit references relative to the start of
the table, and the destination address can be obtained by adding the
two.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/crypto/crc32c-pcl-intel-asm_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
index bbcff1fb78cb..45b005935194 100644
--- a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
+++ b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
@@ -53,7 +53,7 @@
 .endm

 .macro JMPTBL_ENTRY i
-.quad .Lcrc_\i
+.long .Lcrc_\i - jump_table
 .endm

 .macro JNC_LESS_THAN j
@@ -169,7 +169,8 @@ SYM_FUNC_START(crc_pcl)

 	## branch into array
 	leaq	jump_table(%rip), %bufp
-	mov	(%bufp,%rax,8), %bufp
+	movslq	(%bufp,%rax,4), len
+	addq	len, %bufp
 	JMP_NOSPEC bufp

 	################################################################
-- 
2.47.0.rc1.288.g06298d1525-goog