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core/pkg-utils: add macro to escape-and-printf

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In some cases we need to escape make variables and pass them to
printf(1).

This is the case in our fs infra, where we want to shoe-horn the
commands to generate the filesystems in the fakeroot script, or the
devices, permissions and users tables to their respective files.

We currently do so by replacing $(sep) with the literal '\n' but that's
not enough. This does not protect against strings with an embedded '%'
or a backslash.

Add a new macro that properly escapes a string and calls printf(1), so
that we get the expected output.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
This commit is contained in:
Yann E. MORIN 2016-06-06 22:43:38 +02:00 committed by Thomas Petazzoni
parent 77f4c205b9
commit cc05f407bc
1 changed files with 37 additions and 0 deletions
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37
package/pkg-utils.mk
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@ -104,6 +104,43 @@ define sep
endef
PERCENT = %
QUOTE = '
# ' # Meh... syntax-highlighting
# This macro properly escapes a command string, then prints it with printf:
#
# - first, backslash '\' are self-escaped, so that they do not escape
# the following char and so that printf properly outputs a backslash;
#
# - next, single quotes are escaped by closing an existing one, adding
# an escaped one, and re-openning a new one (see below for the reason);
#
# - then '%' signs are self-escaped so that the printf does not interpret
# them as a format specifier, in case the variable contains an actual
# printf with a format;
#
# - finally, $(sep) is replaced with the literal '\n' so that make does
# not break on the so-expanded variable, but so that the printf does
# correctly output an LF.
#
# Note: this must be escaped in this order to avoid over-escaping the
# previously escaped elements.
#
# Once everything has been escaped, it is passed between single quotes
# (that's why the single-quotes are escaped they way they are, above,
# and why the dollar sign is not escaped) to printf(1). A trailing
# newline is apended, too.
#
# Note: leading or trailing spaces are *not* stripped.
#
define PRINTF
printf '$(subst $(sep),\n,\
$(subst $(PERCENT),$(PERCENT)$(PERCENT),\
$(subst $(QUOTE),$(QUOTE)\$(QUOTE)$(QUOTE),\
$(subst \,\\,$(1)))))\n'
endef
# check-deprecated-variable -- throw an error on deprecated variables
# example:
# $(eval $(call check-deprecated-variable,FOO_MAKE_OPT,FOO_MAKE_OPTS))