On Tue, Apr 13, 2021 at 04:03:19PM +0100, Stefan Hajnoczi wrote:
> The size of C arrays at file scope must be constant. The following
> compiler error occurs with recent upstream glibc (2.33.9000):
>
> CC ucontext-cp
> ucontext-cp.c:31:23: error: variably modified ‘stack_buf’ at file scope
> 31 | unsigned char stack_buf[SIGSTKSZ];
> | ^~~~~~~~~
> make[1]: *** [Makefile:26: ucontext-cp] Error 1
>
> The following glibc commit changed SIGSTKSZ from a constant value to a
> variable:
>
> commit 6c57d320484988e87e446e2e60ce42816bf51d53
> Author: H.J. Lu <hjl.tools@xxxxxxxxx>
> Date: Mon Feb 1 11:00:38 2021 -0800
>
> sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]
> ...
> +# define SIGSTKSZ sysconf (_SC_SIGSTKSZ)
>
> Allocate the stack buffer explicitly to avoid declaring an array at file
> scope.
>
> Cc: H.J. Lu <hjl.tools@xxxxxxxxx>
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> ---
> Perhaps the glibc change needs to be revised before releasing glibc 2.34
> since it might break applications. That's up to the glibc folks. It
> doesn't hurt for liburing to take a safer approach that copes with the
> SIGSTKSZ change in any case.
glibc folks, please take a look. The commit referenced above broke
compilation of liburing's tests. It's possible that applications will
hit similar issues. Can you check whether the SIGSTKSZ change needs to
be reverted/fixed before releasing glibc 2.34?
Thanks,
Stefan
> ---
> examples/ucontext-cp.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/examples/ucontext-cp.c b/examples/ucontext-cp.c
> index 0b2a6b5..ea0c934 100644
> --- a/examples/ucontext-cp.c
> +++ b/examples/ucontext-cp.c
> @@ -28,7 +28,7 @@
>
> typedef struct {
> struct io_uring *ring;
> - unsigned char stack_buf[SIGSTKSZ];
> + unsigned char *stack_buf;
> ucontext_t ctx_main, ctx_fnew;
> } async_context;
>
> @@ -115,8 +115,13 @@ static int setup_context(async_context *pctx, struct io_uring *ring)
> perror("getcontext");
> return -1;
> }
> - pctx->ctx_fnew.uc_stack.ss_sp = &pctx->stack_buf;
> - pctx->ctx_fnew.uc_stack.ss_size = sizeof(pctx->stack_buf);
> + pctx->stack_buf = malloc(SIGSTKSZ);
> + if (!pctx->stack_buf) {
> + perror("malloc");
> + return -1;
> + }
> + pctx->ctx_fnew.uc_stack.ss_sp = pctx->stack_buf;
> + pctx->ctx_fnew.uc_stack.ss_size = SIGSTKSZ;
> pctx->ctx_fnew.uc_link = &pctx->ctx_main;
>
> return 0;
> @@ -174,6 +179,7 @@ static void copy_file_wrapper(arguments_bundle *pbundle)
> free(iov.iov_base);
> close(pbundle->infd);
> close(pbundle->outfd);
> + free(pbundle->pctx->stack_buf);
> free(pbundle->pctx);
> free(pbundle);
>
> --
> 2.30.2
>
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