[Buildroot] [PATCH] package/elf2flt: add patch to support SOURCE_DATE_EPOCH

* [Buildroot] [PATCH] package/elf2flt: add patch to support SOURCE_DATE_EPOCH
@ 2019-12-27 17:22 Thomas Petazzoni
  2019-12-29  9:12 ` Yann E. MORIN
  0 siblings, 1 reply; 2+ messages in thread
From: Thomas Petazzoni @ 2019-12-27 17:22 UTC (permalink / raw)
  To: buildroot

The bFLT binaries embed a build_date field in their header, which
doesn't work well with reproducible builds. This commit modifies
elf2flt to support the SOURCE_DATE_EPOCH environment variable.

With this commit in place:

$ hexdump -C output/target/usr/bin/4th | head
00000000  62 46 4c 54 00 00 00 04  00 00 00 45 00 00 ca 40  |bFLT.......E...@|
00000010  00 01 56 28 00 01 9c 10  00 00 10 00 00 01 56 28  |..V(..........V(|
00000020  00 00 05 14 00 00 00 01  5e 06 32 e5 00 00 00 00  |........^.2.....|

$ hexdump -C output/target/sbin/hdparm | head
00000000  62 46 4c 54 00 00 00 04  00 00 00 45 00 01 37 54  |bFLT.......E..7T|
00000010  00 02 0b b0 00 02 79 30  00 00 10 00 00 02 0b b0  |......y0........|
00000020  00 00 07 4c 00 00 00 01  5e 06 32 e5 00 00 00 00  |...L....^.2.....|

So the time encoded is 5e0632e5 for both binaries, which looks
good. And indeed, it matches the time indicated by SOURCE_DATE_EPOCH:

$ make VARS=SOURCE_DATE_EPOCH printvars
SOURCE_DATE_EPOCH=1577464549

$ python -c "print hex(1577464549)"
0x5e0632e5

Fixes:

  http://autobuild.buildroot.net/results/194/194d52402a3de58fc5a502b35b42faa965b40153/diffoscope-results.txt

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
---
 ....c-add-support-for-SOURCE_DATE_EPOCH.patch | 67 +++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 package/elf2flt/0003-elf2flt.c-add-support-for-SOURCE_DATE_EPOCH.patch

diff --git a/package/elf2flt/0003-elf2flt.c-add-support-for-SOURCE_DATE_EPOCH.patch b/package/elf2flt/0003-elf2flt.c-add-support-for-SOURCE_DATE_EPOCH.patch
new file mode 100644
index 0000000000..d25f584879
--- /dev/null
+++ b/package/elf2flt/0003-elf2flt.c-add-support-for-SOURCE_DATE_EPOCH.patch
@@ -0,0 +1,67 @@
+From 0adc7d305055eb88cfb46a203ae39d4d1c82d47e Mon Sep 17 00:00:00 2001
+From: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
+Date: Fri, 27 Dec 2019 16:40:42 +0100
+Subject: [PATCH] elf2flt.c: add support for SOURCE_DATE_EPOCH
+
+The bFLT header has a "build_date" field which contains the date/time
+at which the bFLT file was created. Unfortunately, this breaks
+reproducible builds as two identical builds done at different times
+will produce different results.
+
+For example, on a bFLT binary, diffoscope reports the following
+change:
+
+  00000000: 6246 4c54 0000 0004 0000 0045 0001 48d4  bFLT.......E..H.
+  00000010: 0002 85a0 0002 fe50 0000 1000 0002 85a0  .......P........
+ -00000020: 0000 0757 0000 0001 5e05 742e 0000 0000  ...W....^.t.....
+ +00000020: 0000 0757 0000 0001 5e05 a5c2 0000 0000  ...W....^.......
+
+In order to address this, this commit adds support for the
+SOURCE_DATE_EPOCH environment variable, which is standardized by the
+reproducible-builds.org group at
+https://reproducible-builds.org/specs/source-date-epoch/.
+
+We simply use the time from the SOURCE_DATE_EPOCH variable (which
+contains the number of seconds since Epoch) when SOURCE_DATE_EPOCH is
+available in the environment, and otherwise fallback to the existing
+logic that takes the current time using time().
+
+Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
+Upstream: https://github.com/uclinux-dev/elf2flt/pull/13
+---
+ elf2flt.c | 12 +++++++++++-
+ 1 file changed, 11 insertions(+), 1 deletion(-)
+
+diff --git a/elf2flt.c b/elf2flt.c
+index 8973cef..cc8a1e0 100644
+--- a/elf2flt.c
++++ b/elf2flt.c
+@@ -1721,6 +1721,9 @@ int main(int argc, char *argv[])
+ 
+   struct flat_hdr hdr;
+ 
++  char *sde;
++  time_t t;
++
+   elf2flt_progname = argv[0];
+   xmalloc_set_program_name(elf2flt_progname);
+ 
+@@ -1948,7 +1951,14 @@ int main(int argc, char *argv[])
+ 	  | (pic_with_got ? FLAT_FLAG_GOTPIC : 0)
+ 	  | (docompress ? (docompress == 2 ? FLAT_FLAG_GZDATA : FLAT_FLAG_GZIP) : 0)
+ 	  );
+-  hdr.build_date = htonl((uint32_t)time(NULL));
++
++  sde = getenv("SOURCE_DATE_EPOCH");
++  if (sde)
++      t = (time_t) strtoul(sde, NULL, 10);
++  else
++      t = time(NULL);
++  hdr.build_date = htonl((uint32_t)t);
++
+   memset(hdr.filler, 0x00, sizeof(hdr.filler));
+ 
+   for (i=0; i<reloc_len; i++) reloc[i] = htonl(reloc[i]);
+-- 
+2.24.1
+
-- 
2.24.1

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