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From: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
To: Marc Lehmann <debian-reportbug@plan9.de>, 681612-done@bugs.debian.org
Subject: Re: Bug#681612: libssl1.0.0: fishy reliance on errno when errno is
 not set
Message-ID: <20260321133609.sYfQ1WeV@breakpoint.cc>
References: <20120714183008.4181.29861.reportbug@cerebro.laendle>
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In-Reply-To: <20120714183008.4181.29861.reportbug@cerebro.laendle>

On 2012-07-14 20:30:08 [+0200], Marc Lehmann wrote:
> Hi!
Hi,

> Specifically, what tipped me of is this macro:
> 
>    #define clear_sys_error()      errno=0
> 
> It is used a great number of times in openssl, so the problem I saw might
> be endemic. The above macro cannot be of use in a POSIX (or GNU/Linux
> program), except for very few functions that require it (none of which
> openssl uses) or possibly to improve the debugging experience by having
> errno 0 "more often" in the debugger.
> 
> However, openssl wrongly relies on it, at least in ./crypto/bio/bss_fd.c:
> 
> fd_read calls read(), and if read returns <= 0, it calls BIO_fd_should_retry.
> 
> BIO_fd_should_retry sets BIO_fd_non_fatal_error with errno if ret == 0.
> 
> However, when read returns 0, errno has undefined contents (0 is not a
> failure, and errno is only defined after a failure), so openssl relies on
> an undefined value in BIO_fd_non_fatal_error which in turn might make it
> signal an error when none happened (or worse).

if read() returns 0 then errno is unchanged. It was set to 0 so it
should remain 0. From looking in the man-page, there no indication of
undefined behaviour, it says "return -1 and errno indicates the error".

Therefore closing. If you think this is wrong, please come back.

Sebastian]