rtl apps crash "allocating zero-copy buffers", Pi 4, Ubuntu 20.04

[Edward Vielmetti]

There is a discussion at https://www.reddit.com/r/RTLSDR/comments/ds73ow/rtl_process_being_killed_newbie_troubleshooting/
for reference.

When installed with the default librtlsdr-dev, the rtl apps crash
on Ubuntu 20.04 LTS with this error sequence. This example is from
rtl-test:

Reading samples in async mode...
Allocating 15 zero-copy buffers
Killed

The relevant patch from upstream is

https://github.com/steve-m/librtlsdr/commit/81833a1cf6288fee93a9157c0f60cafb5ec340b9

lib: disable usbfs zero-copy support by default

Although we added a detection mechanism for the presence of the Kernel
bug earlier, reading from the incorrectly mapped memory might cause a
bus error on some ARM systems.

With the overall performance benefit being rather minimal for the
data rates of rtl-sdr, disable zero-copy by default.

[Edward Vielmetti (edward-vielmetti)]

To be more precise, this is happening on a Pi 4 running arm64 (64-bit Arm). The upstream patch mentions "some Arm systems".

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[nyanpasu64 (nyanpasu64)]

Same bug on a Pi 3B running Ubuntu 20.04 arm64.

https://www.reddit.com/r/RTLSDR/comments/ds73ow/rtl_process_being_killed_newbie_troubleshooting/f6ynsaw/ proposes a patch for this bug:

> Reason for this is a bug in librtlsdr. It detects a kernel/hw whithout mmap bug, but crashes in code that uses it. I have made a quickpatch for this.

--- rtl-sdr-0.6.orig/src/librtlsdr.c +++ rtl-sdr-0.6/src/librtlsdr.c
@@ -1748,7 +1748,7 @@ static int _rtlsdr_alloc_async_buffers(r
        dev->xfer_buf = malloc(dev->xfer_buf_num * sizeof(unsigned char *));
        memset(dev->xfer_buf, 0, dev->xfer_buf_num * sizeof(unsigned char *));

-#if defined (__linux__) && LIBUSB_API_VERSION >= 0x01000105
+#if defined (__linux__) && LIBUSB_API_VERSION >= 0x01000105 && 0
        fprintf(stderr, "Allocating %d zero-copy buffers\n", dev->xfer_buf_num);

        dev->use_zerocopy = 1;