[PATCH] EDAC/thunderx: Replace strlcat() with snprintf() and seq_buf

[Ian Bridges <icb@fastmail.org>]

In preparation for removing the strlcat() API[1], replace its uses in
the error message construction of the ThunderX OCX and L2C threaded
IRQ handlers.

The OCX link and L2C handlers append a single decoded-register string,
so the whole message can be produced with one snprintf(). The OCX com
handler accumulates a variable number of fragments in a per-lane loop,
so use a struct seq_buf, which tracks the current write position and
remaining space internally. seq_buf_str() keeps the message
NUL-terminated for edac_device_handle_ce().

Link: https://github.com/KSPP/linux/issues/370 [1]
Signed-off-by: Ian Bridges <icb@fastmail.org>
---
The seq_buf conversion introduces one behavioral difference on
overflow. strlcat() copied as much of a fragment as would fit, but
seq_buf_puts() is all-or-nothing. A message longer than
OCX_MESSAGE_SIZE therefore truncates at a fragment boundary instead of
mid-fragment. The message was truncated either way.

I don't have ThunderX hardware, so I tested the patch with:
 - arm64 cross-build at W=1 with no warnings. Applies cleanly on
   v7.2-rc1.
 - Module load/unload in an arm64 QEMU guest.
 - A userspace harness comparing old and new message construction;
   outputs matched across ~500k register patterns.

 drivers/edac/thunderx_edac.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/drivers/edac/thunderx_edac.c b/drivers/edac/thunderx_edac.c
index 75c04dfc3962..5210ca18473d 100644
--- a/drivers/edac/thunderx_edac.c
+++ b/drivers/edac/thunderx_edac.c
@@ -20,6 +20,7 @@
 #include <linux/atomic.h>
 #include <linux/bitfield.h>
 #include <linux/circ_buf.h>
+#include <linux/seq_buf.h>
 
 #include <asm/page.h>
 
@@ -1108,6 +1109,7 @@ static irqreturn_t thunderx_ocx_com_threaded_isr(int irq, void *irq_id)
 	int lane;
 	char *msg;
 	char *other;
+	struct seq_buf sb;
 
 	msg = kmalloc(OCX_MESSAGE_SIZE, GFP_KERNEL);
 	other = kmalloc(OCX_OTHER_SIZE, GFP_KERNEL);
@@ -1121,13 +1123,14 @@ static irqreturn_t thunderx_ocx_com_threaded_isr(int irq, void *irq_id)
 				ARRAY_SIZE(ocx->com_err_ctx));
 		ctx = &ocx->com_err_ctx[tail];
 
-		snprintf(msg, OCX_MESSAGE_SIZE, "%s: OCX_COM_INT: %016llx",
-			ocx->edac_dev->ctl_name, ctx->reg_com_int);
+		seq_buf_init(&sb, msg, OCX_MESSAGE_SIZE);
+		seq_buf_printf(&sb, "%s: OCX_COM_INT: %016llx",
+			       ocx->edac_dev->ctl_name, ctx->reg_com_int);
 
 		decode_register(other, OCX_OTHER_SIZE,
 				ocx_com_errors, ctx->reg_com_int);
 
-		strlcat(msg, other, OCX_MESSAGE_SIZE);
+		seq_buf_puts(&sb, other);
 
 		for (lane = 0; lane < OCX_RX_LANES; lane++)
 			if (ctx->reg_com_int & BIT(lane)) {
@@ -1136,16 +1139,17 @@ static irqreturn_t thunderx_ocx_com_threaded_isr(int irq, void *irq_id)
 					 lane, ctx->reg_lane_int[lane],
 					 lane, ctx->reg_lane_stat11[lane]);
 
-				strlcat(msg, other, OCX_MESSAGE_SIZE);
+				seq_buf_puts(&sb, other);
 
 				decode_register(other, OCX_OTHER_SIZE,
 						ocx_lane_errors,
 						ctx->reg_lane_int[lane]);
-				strlcat(msg, other, OCX_MESSAGE_SIZE);
+				seq_buf_puts(&sb, other);
 			}
 
 		if (ctx->reg_com_int & OCX_COM_INT_CE)
-			edac_device_handle_ce(ocx->edac_dev, 0, 0, msg);
+			edac_device_handle_ce(ocx->edac_dev, 0, 0,
+					      seq_buf_str(&sb));
 
 		ocx->com_ring_tail++;
 	}
@@ -1203,15 +1207,13 @@ static irqreturn_t thunderx_ocx_lnk_threaded_isr(int irq, void *irq_id)
 
 		ctx = &ocx->link_err_ctx[tail];
 
-		snprintf(msg, OCX_MESSAGE_SIZE,
-			 "%s: OCX_COM_LINK_INT[%d]: %016llx",
-			 ocx->edac_dev->ctl_name,
-			 ctx->link, ctx->reg_com_link_int);
-
 		decode_register(other, OCX_OTHER_SIZE,
 				ocx_com_link_errors, ctx->reg_com_link_int);
 
-		strlcat(msg, other, OCX_MESSAGE_SIZE);
+		snprintf(msg, OCX_MESSAGE_SIZE,
+			 "%s: OCX_COM_LINK_INT[%d]: %016llx%s",
+			 ocx->edac_dev->ctl_name,
+			 ctx->link, ctx->reg_com_link_int, other);
 
 		if (ctx->reg_com_link_int & OCX_COM_LINK_INT_UE)
 			edac_device_handle_ue(ocx->edac_dev, 0, 0, msg);
@@ -1882,14 +1884,12 @@ static irqreturn_t thunderx_l2c_threaded_isr(int irq, void *irq_id)
 
 	while (CIRC_CNT(l2c->ring_head, l2c->ring_tail,
 			ARRAY_SIZE(l2c->err_ctx))) {
-		snprintf(msg, L2C_MESSAGE_SIZE,
-			 "%s: %s: %016llx, %s: %016llx",
-			 l2c->edac_dev->ctl_name, reg_int_name, ctx->reg_int,
-			 ctx->reg_ext_name, ctx->reg_ext);
-
 		decode_register(other, L2C_OTHER_SIZE, l2_errors, ctx->reg_int);
 
-		strlcat(msg, other, L2C_MESSAGE_SIZE);
+		snprintf(msg, L2C_MESSAGE_SIZE,
+			 "%s: %s: %016llx, %s: %016llx%s",
+			 l2c->edac_dev->ctl_name, reg_int_name, ctx->reg_int,
+			 ctx->reg_ext_name, ctx->reg_ext, other);
 
 		if (ctx->reg_int & mask_ue)
 			edac_device_handle_ue(l2c->edac_dev, 0, 0, msg);
-- 
2.47.3