[Qemu-devel] [PATCH v2 1/3] hw/acpi: Replace strncpy() by strpadcpy(pad='\0')

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

From: Marc-André Lureau <marcandre.lureau@redhat.com>

GCC 8 added a -Wstringop-truncation warning:

  The -Wstringop-truncation warning added in GCC 8.0 via r254630 for
  bug 81117 is specifically intended to highlight likely unintended
  uses of the strncpy function that truncate the terminating NUL
  character from the source string.

This new warning leads to compilation failures:

    CC      hw/acpi/core.o
  In function 'acpi_table_install', inlined from 'acpi_table_add' at qemu/hw/acpi/core.c:296:5:
  qemu/hw/acpi/core.c:184:9: error: 'strncpy' specified bound 4 equals destination size [-Werror=stringop-truncation]
           strncpy(ext_hdr->sig, hdrs->sig, sizeof ext_hdr->sig);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  make: *** [qemu/rules.mak:69: hw/acpi/core.o] Error 1

The ACPI tables don't require the strings to be NUL-terminated,
therefore strncpy is the right function to use here.

We could add a #pragma GCC diagnostic ignored "-Wstringop-truncation"
around, disable the warning globally using -Wno-stringop-truncation,
but since QEMU provides the strpadcpy() which does the same purpose,
simply use it to avoid the annoying warning.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: reword commit subject and description]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 hw/acpi/aml-build.c |  6 ++++--
 hw/acpi/core.c      | 13 +++++++------
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/hw/acpi/aml-build.c b/hw/acpi/aml-build.c
index 1e43cd736d..397833462a 100644
--- a/hw/acpi/aml-build.c
+++ b/hw/acpi/aml-build.c
@@ -24,6 +24,7 @@
 #include "hw/acpi/aml-build.h"
 #include "qemu/bswap.h"
 #include "qemu/bitops.h"
+#include "qemu/cutils.h"
 #include "sysemu/numa.h"
 
 static GArray *build_alloc_array(void)
@@ -1532,13 +1533,14 @@ build_header(BIOSLinker *linker, GArray *table_data,
     h->revision = rev;
 
     if (oem_id) {
-        strncpy((char *)h->oem_id, oem_id, sizeof h->oem_id);
+        strpadcpy((char *)h->oem_id, sizeof h->oem_id, oem_id, '\0');
     } else {
         memcpy(h->oem_id, ACPI_BUILD_APPNAME6, 6);
     }
 
     if (oem_table_id) {
-        strncpy((char *)h->oem_table_id, oem_table_id, sizeof(h->oem_table_id));
+        strpadcpy((char *)h->oem_table_id, sizeof(h->oem_table_id),
+                  oem_table_id, '\0');
     } else {
         memcpy(h->oem_table_id, ACPI_BUILD_APPNAME4, 4);
         memcpy(h->oem_table_id + 4, sig, 4);
diff --git a/hw/acpi/core.c b/hw/acpi/core.c
index aafdc61648..6e8f4e5713 100644
--- a/hw/acpi/core.c
+++ b/hw/acpi/core.c
@@ -31,6 +31,7 @@
 #include "qapi/qapi-visit-misc.h"
 #include "qemu/error-report.h"
 #include "qemu/option.h"
+#include "qemu/cutils.h"
 
 struct acpi_table_header {
     uint16_t _length;         /* our length, not actual part of the hdr */
@@ -181,7 +182,7 @@ static void acpi_table_install(const char unsigned *blob, size_t bloblen,
     ext_hdr->_length = cpu_to_le16(acpi_payload_size);
 
     if (hdrs->has_sig) {
-        strncpy(ext_hdr->sig, hdrs->sig, sizeof ext_hdr->sig);
+        strpadcpy(ext_hdr->sig, sizeof ext_hdr->sig, hdrs->sig, '\0');
         ++changed_fields;
     }
 
@@ -200,12 +201,12 @@ static void acpi_table_install(const char unsigned *blob, size_t bloblen,
     ext_hdr->checksum = 0;
 
     if (hdrs->has_oem_id) {
-        strncpy(ext_hdr->oem_id, hdrs->oem_id, sizeof ext_hdr->oem_id);
+        strpadcpy(ext_hdr->oem_id, sizeof ext_hdr->oem_id, hdrs->oem_id, '\0');
         ++changed_fields;
     }
     if (hdrs->has_oem_table_id) {
-        strncpy(ext_hdr->oem_table_id, hdrs->oem_table_id,
-                sizeof ext_hdr->oem_table_id);
+        strpadcpy(ext_hdr->oem_table_id, sizeof ext_hdr->oem_table_id,
+                  hdrs->oem_table_id, '\0');
         ++changed_fields;
     }
     if (hdrs->has_oem_rev) {
@@ -213,8 +214,8 @@ static void acpi_table_install(const char unsigned *blob, size_t bloblen,
         ++changed_fields;
     }
     if (hdrs->has_asl_compiler_id) {
-        strncpy(ext_hdr->asl_compiler_id, hdrs->asl_compiler_id,
-                sizeof ext_hdr->asl_compiler_id);
+        strpadcpy(ext_hdr->asl_compiler_id, sizeof ext_hdr->asl_compiler_id,
+                  hdrs->asl_compiler_id, '\0');
         ++changed_fields;
     }
     if (hdrs->has_asl_compiler_rev) {
-- 
2.17.2