[PATCH] lra: set insn_code_data to NULL when freeing

[Vladimir Makarov]

On 2020-03-30 12:06 p.m., David Malcolm wrote:
> It's a double-free bug in lra.c, albeit one that requires being used
> in a multithreaded way from libgccjit to be triggered.
>
> libgccjit's test-threads.c repeatedly compiles and runs numerous tests,
> each in a separate thread.
>
> Attempting to add an empty test that generates no code leads to a
> double-free ICE within that thread, within lra.c's
> finish_insn_code_data_once.
>
> The root cause is that the insn_code_data array is cleared in
> init_insn_code_data_once, but this is only called the first time
> a cgraph_node is expanded [1], whereas the "loop-over-all-elements
> and free them" is unconditionally called in finalize [2].  Hence
> if there are no functions:
> * the array is not re-initialized for the empty context
> * when finish_insn_code_data_once is called for the empty context
> it still contains the freed pointers from the previous context
> that held the jit mutex, and hence the free is a double-free.
>
> This patch sets the pointers to NULL after freeing them, fixing
> the ICE.  The calls to free are still guarded by a check for NULL,
> which is redundant, but maybe there's a reason for not wanting to
> call "free" on a possibly-NULL value many times on process exit?
> (it makes the diff cleaner, at least)
>
> Fixes the issue in jit.dg.
>
> Full bootstrap & regression test in progress.
>
> Is it OK for master if it passes?

Sure, David.  Thank you for the patch.

> gcc/ChangeLog:
> 	* lra.c (finish_insn_code_data_once): Set the array elements
> 	to NULL after freeing them.
>
> gcc/testsuite/ChangeLog:
> 	* jit.dg/all-non-failing-tests.h: Add test-empty.c
> ---
>   gcc/lra.c                                    |  5 ++++-
>   gcc/testsuite/jit.dg/all-non-failing-tests.h | 10 ++++++++++
>   2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/lra.c b/gcc/lra.c
> index d5ea3622686..5e8b75b1fda 100644
> --- a/gcc/lra.c
> +++ b/gcc/lra.c
> @@ -653,7 +653,10 @@ finish_insn_code_data_once (void)
>     for (unsigned int i = 0; i < NUM_INSN_CODES; i++)
>       {
>         if (insn_code_data[i] != NULL)
> -	free (insn_code_data[i]);
> +	{
> +	  free (insn_code_data[i]);
> +	  insn_code_data[i] = NULL;
> +	}
>       }
>   }
>   
> diff --git a/gcc/testsuite/jit.dg/all-non-failing-tests.h b/gcc/testsuite/jit.dg/all-non-failing-tests.h
> index b2acc74ae95..af744192a73 100644
> --- a/gcc/testsuite/jit.dg/all-non-failing-tests.h
> +++ b/gcc/testsuite/jit.dg/all-non-failing-tests.h
> @@ -116,6 +116,13 @@
>   #undef create_code
>   #undef verify_code
>   
> +/* test-empty.c */
> +#define create_code create_code_empty
> +#define verify_code verify_code_empty
> +#include "test-empty.c"
> +#undef create_code
> +#undef verify_code
> +
>   /* test-error-*.c: We don't use these test cases, since they deliberately
>      introduce errors, which we don't want here.  */
>   
> @@ -328,6 +335,9 @@ const struct testcase testcases[] = {
>     {"expressions",
>      create_code_expressions,
>      verify_code_expressions},
> +  {"empty",
> +   create_code_empty,
> +   verify_code_empty},
>     {"factorial",
>      create_code_factorial,
>      verify_code_factorial},