Re: [PATCH] Invalidate auxv cache before creating a core target

[John Baldwin <jhb@FreeBSD.org>]

On 9/20/22 1:28 PM, Luis Machado wrote:
> While adding support for MTE corefiles and running the MTE corefile tests,
> I noticed a strange situation where loading the symbol file + core file
> through the command line has a different behavior compared to firing up
> GDB, loading the symbol file with the "file" command and then loading the
> core file with the "core" command.
> 
> I tracked this down to gdb/auxv.c:get_auxv_inferior_data returning empty
> auxv data for pid 0, which gets cached. This is triggered by attempting to
> read auxv data for the exec target.
> 
> In the early stages of reading the core file, we're still using inferior pid
> 0, so when we attempt to read auxv to determine corefile features, we get the
> cached empty data vector again. This breaks core_gdbarch setup.
> 
> To overcome this, make sure we invalidate the auxv data when creating a new core
> target.
> ---
>   gdb/auxv.c    |  4 ++--
>   gdb/auxv.h    |  2 ++
>   gdb/corelow.c | 15 +++++++++++++++
>   3 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/auxv.c b/gdb/auxv.c
> index 6154988f6dd..2423f51f224 100644
> --- a/gdb/auxv.c
> +++ b/gdb/auxv.c
> @@ -342,9 +342,9 @@ invalidate_auxv_cache_inf (struct inferior *inf)
>     auxv_inferior_data.clear (inf);
>   }
>   
> -/* Invalidate current inferior's auxv cache.  */
> +/* See auxv.h */
>   
> -static void
> +void
>   invalidate_auxv_cache (void)
>   {
>     invalidate_auxv_cache_inf (current_inferior ());
> diff --git a/gdb/auxv.h b/gdb/auxv.h
> index a4801c34d2f..242bc2eff00 100644
> --- a/gdb/auxv.h
> +++ b/gdb/auxv.h
> @@ -79,5 +79,7 @@ extern int fprint_target_auxv (struct ui_file *file, struct target_ops *ops);
>   
>   extern target_xfer_partial_ftype memory_xfer_auxv;
>   
> +/* Invalidate current inferior's auxv cache.  */
> +void invalidate_auxv_cache (void);
>   
>   #endif
> diff --git a/gdb/corelow.c b/gdb/corelow.c
> index 293bc8d4f59..1c30c7d011c 100644
> --- a/gdb/corelow.c
> +++ b/gdb/corelow.c
> @@ -53,6 +53,7 @@
>   #include "gdbcmd.h"
>   #include "xml-tdesc.h"
>   #include "memtag.h"
> +#include "auxv.h"
>   
>   #ifndef O_LARGEFILE
>   #define O_LARGEFILE 0
> @@ -515,6 +516,20 @@ core_target_open (const char *arg, int from_tty)
>   
>     current_program_space->cbfd = std::move (temp_bfd);
>   
> +  /* Some architectures identify features using the hwcap bits that
> +     come from the auxv.  Before creating a new core target, make sure we
> +     invalidate the auxv cache so we can get fresh data.
> +
> +     This is required when we have an exec target before loading the core
> +     file.  Under such conditions the pid is 0, and the exec target may
> +     attempt to fetch and cache the auxv.  This results in GDB caching
> +     empty contents as the exec target doesn't have auxv data.
> +
> +     Given we haven't determined the pid yet (we will read it later), it can
> +     still be 0 and simply fetching the auxv may return the cached empty
> +     data.  This ultimately results in problems coming up with the proper
> +     gdbarch data.  */
> +  invalidate_auxv_cache ();
>     core_target *target = new core_target ();
>   
>     /* Own the target until it is successfully pushed.  */

This looks good to me.

-- 
John Baldwin