Re: [PATCH] elf/tst-big-note: Improve accuracy of test [BZ #20419]

[Florian Weimer <fweimer at redhat dot com>]

* Paul Pluzhnikov:

> On Fri, Mar 1, 2019 at 6:28 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> It is possible that the link editor injects an allocated ABI tag note
>> before the artificial, allocated large note in the test.
>
> Looks good to me. Thanks!
>
>> +$(objpfx)tst-big-note-lib.so: $(objpfx)tst-big-note-lib.o
>> +       $(LINK.o) -shared -o $@ $(LDFLAGS.so) $<
>
> I got lost tracing through Makefile machinery trying to find how
> -nostartfiles gets into the link line here, but I'll take your word
> for it :-)

I get this command (slightly 

gcc -m32 -nostdlib -nostartfiles -o /BUILD/elf/tst-big-note
  -Wl,-z,combreloc -Wl,-z,relro -Wl,--hash-style=both /BUILD/csu/crt1.o
  /BUILD/csu/crti.o `gcc -m32 --print-file-name=crtbegin.o`
  /BUILD/elf/tst-big-note.o /BUILD/support/libsupport_nonshared.a
  /BUILD/elf/tst-big-note-lib.so -Wl,-dynamic-linker=/lib/ld-linux.so.2
  -Wl,-rpath-link=/BUILD:/BUILD/math:/BUILD/elf:/BUILD/dlfcn:/BUILD/nss:/BUILD/nis:/BUILD/rt:/BUILD/resolv:/BUILD/mathvec:/BUILD/support:/BUILD/crypt:/BUILD/nptl
  /BUILD/libc.so.6 /BUILD/libc_nonshared.a -Wl,--as-needed
  /BUILD/elf/ld.so -Wl,--no-as-needed -lgcc -Wl,--as-needed -lgcc_s
  -Wl,--no-as-needed `gcc -m32 --print-file-name=crtend.o`
  /BUILD/csu/crtn.o

But looking at the build output, I saw new warnings:

Makefile:1522: warning: overriding recipe for target '/home/fweimer/src/gnu/glib
c/build/elf/tst-big-note-lib.so'
../Makerules:769: warning: ignoring old recipe for target
'/home/fweimer/src/gnu/glibc/build/elf/tst-big-note-lib.so'

I missed one further adjustment in the filtmod1 case I needed to mirror,
but with the new patch, the warning is gone.

Thanks,
Florian

elf/tst-big-note: Improve accuracy of test [BZ #20419]

It is possible that the link editor injects an allocated ABI tag note
before the artificial, allocated large note in the test.  Note parsing
in open_verify stops when the first ABI tag note is encountered, so if
the ABI tag note comes first, the problematic code is not actually
exercised.

Also tweak the artificial note so that it is a syntactically valid
4-byte aligned note, in case the link editor tries to parse notes and
process them.

Improves the testing part of commit 0065aaaaae51cd60210ec3a7e13.

2019-03-01  Florian Weimer  <fweimer@redhat.com>

	[BZ #20419]
	* elf/tst-big-note-lib.S: Create a syntactically valid note.
	* elf/Makefile (tst-big-note-lib.so): Do not link with startup
	code, to avoid creating an ABI tag note.
	(modules-names-nobuild): Add tst-big-note-lib.

diff --git a/elf/Makefile b/elf/Makefile
index 55204073a3..310a37cc13 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -316,8 +316,8 @@ endif
 modules-execstack-yes = tst-execstack-mod
 extra-test-objs += $(addsuffix .os,$(strip $(modules-names)))
 
-# filtmod1.so has a special rule
-modules-names-nobuild := filtmod1
+# filtmod1.so, tst-big-note-lib.so have special rules.
+modules-names-nobuild := filtmod1 tst-big-note-lib
 
 tests += $(tests-static)
 
@@ -1515,6 +1515,11 @@ tst-libc_dlvsym-static-ENV = \
 $(objpfx)tst-libc_dlvsym-static.out: $(objpfx)tst-libc_dlvsym-dso.so
 
 $(objpfx)tst-big-note: $(objpfx)tst-big-note-lib.so
+# Avoid creating an ABI tag note, which may come before the
+# artificial, large note in tst-big-note-lib.o and invalidate the
+# test.
+$(objpfx)tst-big-note-lib.so: $(objpfx)tst-big-note-lib.o
+	$(LINK.o) -shared -o $@ $(LDFLAGS.so) $<
 
 $(objpfx)tst-unwind-ctor: $(objpfx)tst-unwind-ctor-lib.so
 
diff --git a/elf/tst-big-note-lib.S b/elf/tst-big-note-lib.S
index e2008cf4ae..721686fa0e 100644
--- a/elf/tst-big-note-lib.S
+++ b/elf/tst-big-note-lib.S
@@ -20,7 +20,13 @@
    On a typical Linux system with 8MiB "ulimit -s", that was enough
    to trigger stack overflow in open_verify.  */
 
+#define NOTE_SIZE 8*1024*1024
+
 .pushsection .note.big,"a"
-.balign 4
-.fill 8*1024*1024, 1, 0
+	.balign 4
+	.long 5 		/* n_namesz.  Length of "GLIBC".  */
+	.long NOTE_SIZE		/* n_descsz.  */
+	.long 0			/* n_type.  */
+	.ascii "GLIBC\0\0\0"	/* Name and alignment to four bytes.  */
+	.fill NOTE_SIZE, 1, 0
 .popsection