[PATCH 6.1.y 1/2] powerpc/pseries: Make caller pass buffer to plpks_read_var()

[Sasha Levin <sashal@kernel.org>]

From: Andrew Donnellan <ajd@linux.ibm.com>

[ Upstream commit 0cf2cc1fe4e2e7a37da077cdd3fba5cfd9a6a36c ]

Currently, plpks_read_var() allocates a buffer to pass to the
H_PKS_READ_OBJECT hcall, then allocates another buffer into which the data
is copied, and returns that buffer to the caller.

This is a bit over the top - while we probably still want to allocate a
separate buffer to pass to the hypervisor in the hcall, we can let the
caller allocate the final buffer and specify the size.

Don't allocate var->data in plpks_read_var(), instead expect the caller to
allocate it. If the caller needs to discover the size, it can set
var->data to NULL and var->datalen will be populated. Update header file
to document this.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-20-ajd@linux.ibm.com
Stable-dep-of: d52a1659cb17 ("pseries/plpks: fix error handling in plpks_read_var()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/powerpc/include/asm/plpks.h       | 12 ++++++++++++
 arch/powerpc/platforms/pseries/plpks.c | 11 ++++-------
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/include/asm/plpks.h b/arch/powerpc/include/asm/plpks.h
index 9e2219b0202db..4652ad319858e 100644
--- a/arch/powerpc/include/asm/plpks.h
+++ b/arch/powerpc/include/asm/plpks.h
@@ -82,16 +82,28 @@ int plpks_remove_var(char *component, u8 varos,
 
 /**
  * Returns the data for the specified os variable.
+ *
+ * Caller must allocate a buffer in var->data with length in var->datalen.
+ * If no buffer is provided, var->datalen will be populated with the object's
+ * size.
  */
 int plpks_read_os_var(struct plpks_var *var);
 
 /**
  * Returns the data for the specified firmware variable.
+ *
+ * Caller must allocate a buffer in var->data with length in var->datalen.
+ * If no buffer is provided, var->datalen will be populated with the object's
+ * size.
  */
 int plpks_read_fw_var(struct plpks_var *var);
 
 /**
  * Returns the data for the specified bootloader variable.
+ *
+ * Caller must allocate a buffer in var->data with length in var->datalen.
+ * If no buffer is provided, var->datalen will be populated with the object's
+ * size.
  */
 int plpks_read_bootloader_var(struct plpks_var *var);
 
diff --git a/arch/powerpc/platforms/pseries/plpks.c b/arch/powerpc/platforms/pseries/plpks.c
index 00137d6a303cd..b66a980478fec 100644
--- a/arch/powerpc/platforms/pseries/plpks.c
+++ b/arch/powerpc/platforms/pseries/plpks.c
@@ -528,17 +528,14 @@ static int plpks_read_var(u8 consumer, struct plpks_var *var)
 		goto out_free_output;
 	}
 
-	if (var->datalen == 0 || var->datalen > retbuf[0])
+	if (!var->data || var->datalen > retbuf[0])
 		var->datalen = retbuf[0];
 
-	var->data = kzalloc(var->datalen, GFP_KERNEL);
-	if (!var->data) {
-		rc = -ENOMEM;
-		goto out_free_output;
-	}
 	var->policy = retbuf[1];
 
-	memcpy(var->data, output, var->datalen);
+	if (var->data)
+		memcpy(var->data, output, var->datalen);
+
 	rc = 0;
 
 out_free_output:
-- 
2.53.0