linux-kernel - [PATCH AUTOSEL 4.14 116/371] powerpc/64s: Fix logic when handling unknown CPU features

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Message-Id: <20200116172403.18149-59-sashal@kernel.org>
Date:   Thu, 16 Jan 2020 12:19:48 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Sasha Levin <sashal@...nel.org>, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH AUTOSEL 4.14 116/371] powerpc/64s: Fix logic when handling unknown CPU features

From: Michael Ellerman <mpe@...erman.id.au>

[ Upstream commit 8cfaf106918a8c13abb24c641556172afbb9545c ]

In cpufeatures_process_feature(), if a provided CPU feature is unknown and
enable_unknown is false, we erroneously print that the feature is being
enabled and return true, even though no feature has been enabled, and
may also set feature bits based on the last entry in the match table.

Fix this so that we only set feature bits from the match table if we have
actually enabled a feature from that table, and when failing to enable an
unknown feature, always print the "not enabling" message and return false.

Coincidentally, some older gccs (<GCC 7), when invoked with
-fsanitize-coverage=trace-pc, cause a spurious uninitialised variable
warning in this function:

  arch/powerpc/kernel/dt_cpu_ftrs.c: In function ‘cpufeatures_process_feature’:
  arch/powerpc/kernel/dt_cpu_ftrs.c:686:7: warning: ‘m’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    if (m->cpu_ftr_bit_mask)

An upcoming patch will enable support for kcov, which requires this option.
This patch avoids the warning.

Fixes: 5a61ef74f269 ("powerpc/64s: Support new device tree binding for discovering CPU features")
Reported-by: Segher Boessenkool <segher@...nel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
[ajd: add commit message]
Signed-off-by: Andrew Donnellan <andrew.donnellan@....ibm.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/powerpc/kernel/dt_cpu_ftrs.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index 2357df60de95..7ed2b1b6643c 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c
@@ -705,8 +705,10 @@ static bool __init cpufeatures_process_feature(struct dt_cpu_feature *f)
 		m = &dt_cpu_feature_match_table[i];
 		if (!strcmp(f->name, m->name)) {
 			known = true;
-			if (m->enable(f))
+			if (m->enable(f)) {
+				cur_cpu_spec->cpu_features |= m->cpu_ftr_bit_mask;
 				break;
+			}
 
 			pr_info("not enabling: %s (disabled or unsupported by kernel)\n",
 				f->name);
@@ -714,17 +716,12 @@ static bool __init cpufeatures_process_feature(struct dt_cpu_feature *f)
 		}
 	}
 
-	if (!known && enable_unknown) {
-		if (!feat_try_enable_unknown(f)) {
-			pr_info("not enabling: %s (unknown and unsupported by kernel)\n",
-				f->name);
-			return false;
-		}
+	if (!known && (!enable_unknown || !feat_try_enable_unknown(f))) {
+		pr_info("not enabling: %s (unknown and unsupported by kernel)\n",
+			f->name);
+		return false;
 	}
 
-	if (m->cpu_ftr_bit_mask)
-		cur_cpu_spec->cpu_features |= m->cpu_ftr_bit_mask;
-
 	if (known)
 		pr_debug("enabling: %s\n", f->name);
 	else
-- 
2.20.1