[Buildroot] [PATCH v2 1/1] qemu: add patch to fix SSP support detection

[Rodrigo Rebello <rprebello@gmail.com>]

The QEMU configure script incorrectly assumes SSP is supported by the
toolchain in some cases where the compiler accepts -fstack-protector-*
flags but the C library does not provide the necessary __stack_chk_*()
functions.

Even though a full compile and link test is performed by the script,
this is done with a code fragment which does not actually meet any of
the conditions required to cause the compiler to emit canary code when
the -fstack-protector-strong variant is used. As no compile or link
failure occurs in this case, a false positive is generated and a
subsequent error is seen when the probe for pthreads is performed.

The fix consists in patching the configure script to use an appropriate
test program for the SSP support checks.

Fixes:

  http://autobuild.buildroot.net/results/efb/efbb4e940543894b8745bb405478a096c90a5ae2/
  http://autobuild.buildroot.net/results/32d/32d6d984febad2dee1f0d31c5fa0aea823297096/
  http://autobuild.buildroot.net/results/aa6/aa6e71c957fb6f07e7bded35a8e47be4dadd042c/
  ...and many others.

Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
---
Changes v1 -> v2:
  - Patch the configure script instead of force disable SSP detection
    (Arnout Vandecappelle).
---
 ...se-appropriate-code-fragment-for-fstack-p.patch | 65 ++++++++++++++++++++++
 1 file changed, 65 insertions(+)
 create mode 100644 package/qemu/0001-configure-use-appropriate-code-fragment-for-fstack-p.patch

diff --git a/package/qemu/0001-configure-use-appropriate-code-fragment-for-fstack-p.patch b/package/qemu/0001-configure-use-appropriate-code-fragment-for-fstack-p.patch
new file mode 100644
index 0000000..5eee141
--- /dev/null
+++ b/package/qemu/0001-configure-use-appropriate-code-fragment-for-fstack-p.patch
@@ -0,0 +1,65 @@
+From 83897ad507f8bb332000304b96d36c109c19bfad Mon Sep 17 00:00:00 2001
+From: Rodrigo Rebello <rprebello@gmail.com>
+Date: Wed, 11 Nov 2015 18:39:24 -0200
+Subject: [PATCH 1/1] configure: use appropriate code fragment for
+ -fstack-protector checks
+Cc: qemu-trivial at nongnu.org
+
+The check for stack-protector support consisted in compiling and linking
+the test program below (output by function write_c_skeleton()) with the
+compiler flag -fstack-protector-strong first and then with
+-fstack-protector-all if the first one failed to work:
+
+  int main(void) { return 0; }
+
+This caused false positives when using certain toolchains in which the
+compiler accepted -fstack-protector-strong but no support was provided
+by the C library, since for this stack-protector variant the compiler
+emits canary code only for functions that meet specific conditions
+(local arrays, memory references to local variables, etc.) and the code
+fragment under test included none of them (hence no stack protection
+code generated, no link failure).
+
+This fix changes the test program used for -fstack-protector checks to
+include a function that meets conditions which cause the compiler to
+generate canary code in all variants.
+
+Upstream status: sent
+
+Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
+---
+ configure | 18 ++++++++++++++++++
+ 1 file changed, 18 insertions(+)
+
+diff --git a/configure b/configure
+index cd219d8..a6f4101 100755
+--- a/configure
++++ b/configure
+@@ -1471,6 +1471,24 @@ for flag in $gcc_flags; do
+ done
+ 
+ if test "$stack_protector" != "no"; then
++  cat > $TMPC << EOF
++void foo(const char *c);
++
++void foo(const char *c)
++{
++    char arr[64], *p;
++    for (p = arr; *c; c++, p++) {
++        *p = *c;
++    }
++}
++
++int main(void)
++{
++    char c[] = "";
++    foo(c);
++    return 0;
++}
++EOF
+   gcc_flags="-fstack-protector-strong -fstack-protector-all"
+   sp_on=0
+   for flag in $gcc_flags; do
+-- 
+2.1.4
+
-- 
2.1.4