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Make __DATE__/__TIME__ deterministic when NIX_ENFORCE_PURITY=1 #51510

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Make __DATE__/__TIME__ deterministic when NIX_ENFORCE_PURITY=1 #51510

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9 changes: 9 additions & 0 deletions pkgs/build-support/cc-wrapper/cc-wrapper.sh
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Expand Up @@ -137,6 +137,15 @@ source @out@/nix-support/add-hardening.sh
extraAfter=($NIX_@infixSalt@_CFLAGS_COMPILE)
extraBefore=(${hardeningCFlags[@]+"${hardeningCFlags[@]}"})

# When enforcing purity, pretend gcc can't find the current date and
# time
if [[ "${NIX_ENFORCE_PURITY:-}" = 1 ]]; then
extraAfter=(-D__DATE__=\"???-??-????\"
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@Mic92 Mic92 Dec 4, 2018

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Did you mean?

Suggested change
extraAfter=(-D__DATE__=\"???-??-????\"
extraAfter=(-D__DATE__=\"??-??-????\"

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@Mic92 Mic92 Dec 4, 2018

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Do you know what debian is doing here?

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@rht rht Dec 4, 2018

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It appears that there has been a patch to gcc (since 2016) to allow using SOURCE_DATE_EPOCH https://wiki.debian.org/ReproducibleBuilds/TimestampsFromCPPMacros.

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@rht rht Dec 4, 2018

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Some packages (e.g. pycrypto) do already have SOURCE_DATE_EPOCH defined, though it would be preferable to do this globally.

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@rht rht Dec 4, 2018

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There is

nixpkgs/pkgs/stdenv/generic/setup.sh

Lines 244 to 249 in 33b9aa4

# Set a fallback default value for SOURCE_DATE_EPOCH, used by some
# build tools to provide a deterministic substitute for the "current"
# time. Note that 1 = 1970-01-01 00:00:01. We don't use 0 because it
# confuses some applications.
export SOURCE_DATE_EPOCH
: ${SOURCE_DATE_EPOCH:=1}
, but not used by all packages built with gcc.

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@Mic92 Mic92 Dec 4, 2018

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But why we need these macros then, when the environment variable is already exported?

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@Mic92 Mic92 Dec 4, 2018

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If a packages ignores environment variables, we are likely detecting this because cc wrapper will also not find libraries and headers.

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@rht rht Dec 4, 2018

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I suppose since the SOURCE_DATE_EPOCH is only supported for gcc >=7.x, but not so for earlier versions, and so I have to modify it to be enabled only for early gcc's.

-D__TIME__=\"??:??:??\"
-Wno-builtin-macro-redefined
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@Mic92 Mic92 Dec 4, 2018

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mhm. Can this hit us somewhere else?

"${extraAfter[@]}")
fi

if [ "$dontLink" != 1 ]; then

# Add the flags that should only be passed to the compiler when
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