linux-kernel - [PATCH] coresight: etm4x: work around clang-12+ build failure

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Date:   Thu, 25 Feb 2021 10:42:58 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH] coresight: etm4x: work around clang-12+ build failure

From: Arnd Bergmann <arnd@...db.de>

clang-12 fails to build the etm4x driver with -fsanitize=array-bounds:

<instantiation>:1:7: error: expected constant expression in '.inst' directive
.inst (0xd5200000|((((2) << 19) | ((1) << 16) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 7) & 0x7)) << 12) | ((((((((((0x160 + (i * 4))))) >> 2))) & 0xf)) << 8) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 4) & 0x7)) << 5)))|(.L__reg_num_x8))
      ^
drivers/hwtracing/coresight/coresight-etm4x-core.c:702:4: note: while in macro instantiation
                        etm4x_relaxed_read32(csa, TRCCNTVRn(i));
                        ^
drivers/hwtracing/coresight/coresight-etm4x.h:403:4: note: expanded from macro 'etm4x_relaxed_read32'
                 read_etm4x_sysreg_offset((offset), false)))
                 ^
drivers/hwtracing/coresight/coresight-etm4x.h:383:12: note: expanded from macro 'read_etm4x_sysreg_offset'
                        __val = read_etm4x_sysreg_const_offset((offset));       \
                                ^
drivers/hwtracing/coresight/coresight-etm4x.h:149:2: note: expanded from macro 'read_etm4x_sysreg_const_offset'
        READ_ETM4x_REG(ETM4x_OFFSET_TO_REG(offset))
        ^
drivers/hwtracing/coresight/coresight-etm4x.h:144:2: note: expanded from macro 'READ_ETM4x_REG'
        read_sysreg_s(ETM4x_REG_NUM_TO_SYSREG((reg)))
        ^
arch/arm64/include/asm/sysreg.h:1108:15: note: expanded from macro 'read_sysreg_s'
        asm volatile(__mrs_s("%0", r) : "=r" (__val));                  \
                     ^
arch/arm64/include/asm/sysreg.h:1074:2: note: expanded from macro '__mrs_s'
"       mrs_s " v ", " __stringify(r) "\n"                      \
 ^

It appears that the __builin_constant_p() check in
read_etm4x_sysreg_offset() falsely returns 'true' here because clang
decides finds that an out-of-bounds access to config->cntr_val[] cannot
happen, and then it unrolls the loop with constant register numbers. Then
when actually emitting the output, it fails to figure out the value again.

While this is incorrect behavior in clang, it is easy to work around
by avoiding the out-of-bounds array access. Do this by limiting the
loop counter to the actual dimension of the array.

Link: https://github.com/ClangBuiltLinux/linux/issues/1310
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/hwtracing/coresight/coresight-etm4x-core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index 15016f757828..4cccf874a602 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -691,13 +691,13 @@ static void etm4_disable_hw(void *info)
 			"timeout while waiting for PM stable Trace Status\n");
 
 	/* read the status of the single shot comparators */
-	for (i = 0; i < drvdata->nr_ss_cmp; i++) {
+	for (i = 0; i < min_t(u32, drvdata->nr_ss_cmp, ETM_MAX_SS_CMP); i++) {
 		config->ss_status[i] =
 			etm4x_relaxed_read32(csa, TRCSSCSRn(i));
 	}
 
 	/* read back the current counter values */
-	for (i = 0; i < drvdata->nr_cntr; i++) {
+	for (i = 0; i < min_t(u32, drvdata->nr_cntr, ETMv4_MAX_CNTR); i++) {
 		config->cntr_val[i] =
 			etm4x_relaxed_read32(csa, TRCCNTVRn(i));
 	}
-- 
2.29.2