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Re: [Qemu-devel] [PATCH v3 3/4] trace: forbid use of %m in trace event f

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 3/4] trace: forbid use of %m in trace event format strings
Date: Tue, 22 Jan 2019 11:25:55 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/22/19 10:49 AM, Daniel P. Berrangé wrote:
> The '%m' format specifier instructs glibc's printf() implementation to
> insert the contents of strerror(errno). Since this is a glibc extension
> it should generally be avoided in QEMU due to need for portability to a
> variety of platforms.

Up to you if you want to mention that syslog() portably supports %m, but
not all trace backends use syslog().

> Even though vfio is Linux-only code that could otherwise use "%m", it
> must still be avoided in trace-events files because several of the
> backends do not use the format string and so this error information is
> invisible to them.
> The errno string value should be given as an explicit trace argument
> instead, making it accessible to all backends. This also allows it to
> work correctly with future patches that use the format string with
> systemtap's simple printf code.
> Signed-off-by: Daniel P. Berrangé <address@hidden>
> ---
>  hw/vfio/pci.c                 | 2 +-
>  hw/vfio/trace-events          | 2 +-
>  scripts/tracetool/__init__.py | 4 ++++
>  3 files changed, 6 insertions(+), 2 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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