Re: [PATCH obv] Remove some superfluous code in corelow.c

[Pedro Alves <palves@redhat.com>]

On 05/04/2017 10:09 AM, Andreas Arnez wrote:
> Pushed this as obvious.
> 
> 
> In corelow.c I stumbled upon an extra semicolon and an xfree of a NULL
> pointer.  Remove them.

> @@ -129,7 +129,7 @@ sniff_core_bfd (bfd *abfd)
>  {
>    struct core_fns *cf;
>    struct core_fns *yummy = NULL;
> -  int matches = 0;;
> +  int matches = 0;
>  
>    /* Don't sniff if we have support for register sets in
>       CORE_GDBARCH.  */
> @@ -511,7 +511,7 @@ get_core_register_section (struct regcache *regcache,
>  			   const char *human_name,
>  			   int required)
>  {
> -  static char *section_name = NULL;
> +  static char *section_name;

But this variable is static..

>    struct bfd_section *section;
>    bfd_size_type size;
>    char *contents;
> @@ -519,8 +519,6 @@ get_core_register_section (struct regcache *regcache,
>  				&& regset->flags & REGSET_VARIABLE_SIZE);
>    ptid_t ptid = regcache_get_ptid (regcache);
>  
> -  xfree (section_name);
> -

..so this was releasing the memory allocated in a previous invocation of
get_core_register_section, AFAICS.  So, this wasn't superfluous,
and we now leak.

I'm not sure why the variable is static but from [1] I suspect 
that the reason was just "laziness" to avoid having to think
about freeing it on every function return path:

 https://sourceware.org/ml/gdb-patches/2005-03/msg00237.html

The easiest to fix that nowadays is to use a std::string.

I don't see a need to xstrdup the section name in the single-threaded
case though, and also there's more than one place that computes
a multi-threaded section name in the same way, so how about the
patch below?

From 7c91ec5530e157c33d6e5edc3dc040d9fba1818b Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Thu, 4 May 2017 10:47:04 +0100
Subject: [PATCH] fix leak, introduce thread_section_name

---
 gdb/corelow.c | 76 +++++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 53 insertions(+), 23 deletions(-)

diff --git a/gdb/corelow.c b/gdb/corelow.c
index 2266f24..0aff02d 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -485,6 +485,51 @@ core_detach (struct target_ops *ops, const char *args, int from_tty)
     printf_filtered (_("No core file now.\n"));
 }
 
+/* Build either a single-thread or multi-threaded section name for
+   PTID.
+
+   If ptid's lwp member is zero, we want to do the single-threaded
+   thing: look for a section named NAME (as passed to the
+   constructor).  If ptid's lwp member is non-zero, we'll want do the
+   multi-threaded thing: look for a section named "NAME/LWP", where
+   LWP is the shortest ASCII decimal representation of ptid's lwp
+   member.  */
+
+class thread_section_name
+{
+public:
+  /* NAME is the single-threaded section name.  If PTID represents an
+     LWP, then the build section name is "NAME/LWP", otherwise it's
+     just "NAME" unmodified.  */
+  thread_section_name (const char *name, ptid_t ptid)
+  {
+    if (ptid.lwp_p ())
+      {
+	m_storage = string_printf ("%s/%ld", name, ptid.lwp ());
+	m_section_name = m_storage.c_str ();
+      }
+    else
+      m_section_name = name;
+  }
+
+  /* Return the computed section name.  The result is valid as long as
+     this thread_section_name object is live.  */
+  const char *c_str () const
+  { return m_section_name; }
+
+  /* Disable copy.  */
+  thread_section_name (const thread_section_name &) = delete;
+  void operator= (const thread_section_name &) = delete;
+
+private:
+  /* Either a pointer into M_STORAGE, or a pointer to the name passed
+     as parameter to the constructor.  */
+  const char *m_section_name;
+  /* If we need to build a new section name, this is where we store
+     it.  */
+  std::string m_storage;
+};
+
 /* Try to retrieve registers from a section in core_bfd, and supply
    them to core_vec->core_read_registers, as the register set numbered
    WHICH.
@@ -511,21 +556,15 @@ get_core_register_section (struct regcache *regcache,
 			   const char *human_name,
 			   int required)
 {
-  static char *section_name;
   struct bfd_section *section;
   bfd_size_type size;
   char *contents;
   bool variable_size_section = (regset != NULL
 				&& regset->flags & REGSET_VARIABLE_SIZE);
-  ptid_t ptid = regcache_get_ptid (regcache);
 
-  if (ptid_get_lwp (ptid))
-    section_name = xstrprintf ("%s/%ld", name,
-			       ptid_get_lwp (ptid));
-  else
-    section_name = xstrdup (name);
+  thread_section_name section_name (name, regcache->ptid ());
 
-  section = bfd_get_section_by_name (core_bfd, section_name);
+  section = bfd_get_section_by_name (core_bfd, section_name.c_str ());
   if (! section)
     {
       if (required)
@@ -537,13 +576,14 @@ get_core_register_section (struct regcache *regcache,
   size = bfd_section_size (core_bfd, section);
   if (size < min_size)
     {
-      warning (_("Section `%s' in core file too small."), section_name);
+      warning (_("Section `%s' in core file too small."),
+	       section_name.c_str ());
       return;
     }
   if (size != min_size && !variable_size_section)
     {
       warning (_("Unexpected size of section `%s' in core file."),
-	       section_name);
+	       section_name.c_str ());
     }
 
   contents = (char *) alloca (size);
@@ -551,7 +591,7 @@ get_core_register_section (struct regcache *regcache,
 				  (file_ptr) 0, size))
     {
       warning (_("Couldn't read %s registers from `%s' section in core file."),
-	       human_name, name);
+	       human_name, section_name.c_str ());
       return;
     }
 
@@ -681,18 +721,8 @@ add_to_spuid_list (bfd *abfd, asection *asect, void *list_p)
 static LONGEST
 get_core_siginfo (bfd *abfd, gdb_byte *readbuf, ULONGEST offset, ULONGEST len)
 {
-  asection *section;
-  char *section_name;
-  const char *name = ".note.linuxcore.siginfo";
-
-  if (ptid_get_lwp (inferior_ptid))
-    section_name = xstrprintf ("%s/%ld", name,
-			       ptid_get_lwp (inferior_ptid));
-  else
-    section_name = xstrdup (name);
-
-  section = bfd_get_section_by_name (abfd, section_name);
-  xfree (section_name);
+  thread_section_name section_name (".note.linuxcore.siginfo", inferior_ptid);
+  asection *section = bfd_get_section_by_name (abfd, section_name.c_str ());
   if (section == NULL)
     return -1;
 
-- 
2.5.5