[PATCH] [PR corefiles/32441] Fix segfault if target_fileio_read_alloc fails

["brandon.belew" <brandon.belew@zetier.com>]

Check for target_fileio_read_alloc failure in linux_fill_prpsinfo
before dereferencing buffer. This fixes a segfault in the 'gcore'
command when attached to certain remote targets.
---
This is my first contribution to GDB, and my first use of
git-send-email, so please let me know if this is formatted
incorrectly! I initially submitted the bug and a v1 patch at
https://sourceware.org/bugzilla/show_bug.cgi?id=32441 and received the
following from Thiago Bauermann:

> Thank you for the patch. In general it looks good to me, just a couple of minor
> comments:
>
> 1. Since target_fileio_read_alloc () returns LONGEST, I think it's better if
> the buf_len variable also has that type.

I decided to stick with ssize_t for the variable, as this matches the
usage elsewhere in linux-tdep.c in linux_info_proc (which already was
correctly checking the length).

> 2. GDB is (very) slowly transitioning from C to C++. We currently prefer to use
> nullptr rather than NULL, so I suggest using this patch as an opportunity to
> change NULL to nullptr in lines 1876, 1877 and 1879.

I made the requested NULL -> nullptr changes.

Let me know if this is good or if I need to make any changes in my
workflow to adhere to GNU or gdb project conventions.

 gdb/linux-tdep.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index d3452059ce2..c10c4c76451 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -1867,17 +1867,17 @@ linux_fill_prpsinfo (struct elf_internal_linux_prpsinfo *p)
   /* The number of fields read by `sscanf'.  */
   int n_fields = 0;

-  gdb_assert (p != NULL);
+  gdb_assert (p != nullptr);

   /* Obtaining PID and filename.  */
   pid = inferior_ptid.pid ();
   xsnprintf (filename, sizeof (filename), "/proc/%d/cmdline", (int) pid);
   /* The full name of the program which generated the corefile.  */
-  gdb_byte *buf = NULL;
-  size_t buf_len = target_fileio_read_alloc (NULL, filename, &buf);
+  gdb_byte *buf = nullptr;
+  ssize_t buf_len = target_fileio_read_alloc (nullptr, filename, &buf);
   gdb::unique_xmalloc_ptr<char> fname ((char *)buf);

-  if (buf_len < 1 || fname.get ()[0] == '\0')
+  if (buf_len < 1 || fname.get () == nullptr || fname.get ()[0] == '\0')
     {
       /* No program name was read, so we won't be able to retrieve more
 	 information about the process.  */
--
2.46.0