gdbserver/tracepoint.cc: work around -Wstringop-truncation error

When building gdb with  on AArch64 with g++ 11.1.0 (and some preceding
versions too), -O2 and -fsanitize=address, I get this error.

      CXX    tracepoint-ipa.o
    cc1plus: warning: command-line option ‘-Wmissing-prototypes’ is valid for C/ObjC but not for C++
    In file included from /usr/include/string.h:519,
                     from ../gnulib/import/string.h:41,
                     from /home/simark/src/binutils-gdb/gdbserver/../gdbsupport/common-defs.h:95,
                     from /home/simark/src/binutils-gdb/gdbserver/server.h:22,
                     from /home/simark/src/binutils-gdb/gdbserver/tracepoint.cc:19:
    In function ‘char* strncpy(char*, const char*, size_t)’,
        inlined from ‘int init_named_socket(const char*)’ at /home/simark/src/binutils-gdb/gdbserver/tracepoint.cc:6902:11,
        inlined from ‘int gdb_agent_socket_init()’ at /home/simark/src/binutils-gdb/gdbserver/tracepoint.cc:6953:26,
        inlined from ‘void* gdb_agent_helper_thread(void*)’ at /home/simark/src/binutils-gdb/gdbserver/tracepoint.cc:7204:41:
    /usr/include/bits/string_fortified.h:95:34: error: ‘char* __builtin_strncpy(char*, const char*, long unsigned int)’ output may be truncated copying 107 bytes from a string of length 107 [-Werror=stringop-truncation]
       95 |   return __builtin___strncpy_chk (__dest, __src, __len,
          |          ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
       96 |                                   __glibc_objsize (__dest));
          |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~

Note that _FORTIFY_SOURCE changes the message a bit, but I get a similar
error if I use -D_FORTIFY_SOURCE=0.

I am pretty sure it's spurious and might be related to this GCC bug:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88780

From what I can see, we are copying from a static 108 bytes long buffer
(the global array agent_socket_name) to a 108 bytes long array,
sockaddr_un::sun_path.  I don't see anything wrong.

Still, it would make things easier if we didn't see this error.  Change
the code to check that the source string length is smaller than the
destination buffer (including space for the NULL byte) and use strcpy.

For anybody who would like to try to reproduce, the full command line
is:

    g++     -I. -I/home/simark/src/binutils-gdb/gdbserver -I/home/simark/src/binutils-gdb/gdbserver/../gdb/regformats -I/home/simark/src/binutils-gdb/gdbserver/.. -I/home/simark/src/binutils-gdb/gdbserver/../include -I/home/simark/src/binutils-gdb/gdbserver/../gdb -I/home/simark/src/binutils-gdb/gdbserver/../gnulib/import -I../gnulib/import -I/home/simark/src/binutils-gdb/gdbserver/.. -I..   -pthread -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-variable -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable -Wno-sign-compare -Wno-error=maybe-uninitialized -Wno-mismatched-tags -Wsuggest-override -Wimplicit-fallthrough=3 -Wduplicated-cond -Wshadow=local -Wdeprecated-copy -Wdeprecated-copy-dtor -Wredundant-move -Wmissing-declarations -Wmissing-prototypes -Wstrict-null-sentinel -Wformat -Wformat-nonliteral -Werror -DGDBSERVER  -DCONFIG_UST_GDB_INTEGRATION -Drpl_strerror_r=strerror_r -Drpl_free=free -fPIC -DIN_PROCESS_AGENT -fvisibility=hidden -g3 -O2 -fsanitize=address   -c -o tracepoint-ipa.o -MT tracepoint-ipa.o -MMD -MP -MF ./.deps/tracepoint-ipa.Tpo /home/simark/src/binutils-gdb/gdbserver/tracepoint.cc

Change-Id: I18e86c0487feead7e7677e69398405e7057cf464

1 files changed, 7 insertions, 2 deletions

diff --git a/gdbserver/tracepoint.cc b/gdbserver/tracepoint.cc
index f176ab2..a62d9a7 100644
--- a/gdbserver/tracepoint.cc
+++ b/gdbserver/tracepoint.cc
@@ -6895,8 +6895,13 @@ init_named_socket (const char *name)
 
   addr.sun_family = AF_UNIX;
 
-  strncpy (addr.sun_path, name, UNIX_PATH_MAX);
-  addr.sun_path[UNIX_PATH_MAX - 1] = '\0';
+  if (strlen (name) >= ARRAY_SIZE (addr.sun_path))
+    {
+      warning ("socket name too long for sockaddr_un::sun_path field: %s", name);
+      return -1;
+    }
+
+  strcpy (addr.sun_path, name);
 
   result = access (name, F_OK);
   if (result == 0)