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* [PATCH liburing] examples/ucontext-cp.c: cope with variable SIGSTKSZ
@ 2021-04-13 15:03 Stefan Hajnoczi
  2021-04-19 14:34 ` Stefan Hajnoczi
  0 siblings, 1 reply; 7+ messages in thread
From: Stefan Hajnoczi @ 2021-04-13 15:03 UTC (permalink / raw)
  To: linux-block; +Cc: libc-alpha, Jens Axboe, Stefan Hajnoczi, H . J . Lu

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@gmail.com>
  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@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
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.
---
 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


^ permalink raw reply related	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2021-04-23 14:13 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-04-13 15:03 [PATCH liburing] examples/ucontext-cp.c: cope with variable SIGSTKSZ Stefan Hajnoczi
2021-04-19 14:34 ` Stefan Hajnoczi
2021-04-19 18:38   ` H.J. Lu
2021-04-22  9:59     ` Stefan Hajnoczi
2021-04-22 14:22       ` Stefano Garzarella
2021-04-23 14:13         ` Pavel Begunkov
2021-04-20  0:05   ` Paul Eggert

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