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Re: [PATCH v10 02/10] qemu-binfmt-conf.sh: enforce safe tests

From: Unai Martinez Corral
Subject: Re: [PATCH v10 02/10] qemu-binfmt-conf.sh: enforce safe tests
Date: Fri, 13 Mar 2020 01:15:08 +0100

Laurent, as Eric explained, this commit (as well as the homogeneisation of spacing) are fixes to pre-existing issues in the script, in order to better match other bash sources in the codebase. It would be possible to pick 1/10 and 2/10 as a separate patchset.

El mar., 10 mar. 2020 a las 12:47, Eric Blake (<address@hidden>) escribió:
On 3/10/20 3:28 AM, Laurent Vivier wrote:
> Le 09/03/2020 à 20:19, Unai Martinez-Corral a écrit :
>> All the tests are prefixed with 'x', in order to avoid risky comparisons
>> (i.e. a user deliberately trying to provoke a syntax error).
> With the quotes I don't see how we can provoke a syntax error.
> Could you provide an example?

Historically, in some shells:

if [ "$foo" = "$bar" ]; then echo hello world; fi

could output 'hello world' (by parsing a parenthesized one-argument
test, and the string '=' is non-empty), but:

if [ "x$foo" = "x$bar" ]; then echo goodbye; fi

did not (since no operator begins with 'x', you have guaranteed the
syntax that [ will parse).  Similarly, if foo=! or foo=-a, you could get
syntax errors (if [ tried to treat the expansion of $foo as an operator
and got thrown off by the remaining arguments not matching an expected

These days, POSIX says that with three arguments when the 2nd is a
binary operator, there is no ambiguity (the binary operator takes
precedence over the ( and ) around the non-empty string test), and
modern bash obeys the POSIX rule without needing the x prefix.  But it
is still better to prefix with x for copy-paste portability to older
shells that do not match current POSIX rules.

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

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