[PATCH 3/3] Fix ubsan build

[Tom Tromey <tromey@adacore.com>]

I tried a build using the undefined behavior sanitizer, and gcc gave
this error:

In file included from /usr/include/string.h:495,
                 from ../gnulib/import/string.h:41,
                 from ../../binutils-gdb/gdb/../gdbsupport/common-defs.h:95,
                 from ../../binutils-gdb/gdb/nat/linux-osdata.c:20:
In function ‘char* strncpy(char*, const char*, size_t)’,
    inlined from ‘void time_from_time_t(char*, int, TIME_T)’ at ../../binutils-gdb/gdb/nat/linux-osdata.c:923:15,
    inlined from ‘void time_from_time_t(char*, int, TIME_T)’ at ../../binutils-gdb/gdb/nat/linux-osdata.c:911:1,
    inlined from ‘void linux_xfer_osdata_sem(buffer*)’ at ../../binutils-gdb/gdb/nat/linux-osdata.c:1082:22:
/usr/include/bits/string_fortified.h:106:34: error: ‘char* __builtin_strncpy(char*, const char*, long unsigned int)’ specified bound 32 equals destination size [-Werror=stringop-truncation]

This patch fixes the problem by subtracting one from the length
parameter to strncpy.

I changed a couple of other similar functions -- gcc does not warn
about these, but I didn't see any substantial difference between the
different cases, and I think these are just latent warnings, to be
triggered in the future by a change to inlining heuristics.

gdb/ChangeLog
2021-05-03  Tom Tromey  <tromey@adacore.com>

	* nat/linux-osdata.c (user_from_uid, time_from_time_t)
	(group_from_gid): Subtract one from strncpy length.
---
 gdb/ChangeLog          | 5 +++++
 gdb/nat/linux-osdata.c | 6 +++---
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/gdb/nat/linux-osdata.c b/gdb/nat/linux-osdata.c
index 7034dd82376..12f66d3c981 100644
--- a/gdb/nat/linux-osdata.c
+++ b/gdb/nat/linux-osdata.c
@@ -212,7 +212,7 @@ user_from_uid (char *user, int maxlen, uid_t uid)
 
   if (pwentry)
     {
-      strncpy (user, pwentry->pw_name, maxlen);
+      strncpy (user, pwentry->pw_name, maxlen - 1);
       /* Ensure that the user name is null-terminated.  */
       user[maxlen - 1] = '\0';
     }
@@ -920,7 +920,7 @@ time_from_time_t (char *time, int maxlen, TIME_T seconds)
 	 characters long.  */
       char buf[30];
       const char *time_str = ctime_r (&t, buf);
-      strncpy (time, time_str, maxlen);
+      strncpy (time, time_str, maxlen - 1);
       time[maxlen - 1] = '\0';
     }
 }
@@ -935,7 +935,7 @@ group_from_gid (char *group, int maxlen, gid_t gid)
 
   if (grentry)
     {
-      strncpy (group, grentry->gr_name, maxlen);
+      strncpy (group, grentry->gr_name, maxlen - 1);
       /* Ensure that the group name is null-terminated.  */
       group[maxlen - 1] = '\0';
     }
-- 
2.26.3