[RFC] Change PCH "checksum"
Richard Biener
rguenther@suse.de
Fri Feb 22 11:29:00 GMT 2019
GCC builds are currently not reproducible because for one the checksum
we compute for PCH purposes (by genchecksum) nowaways includes checksums
of archives (since we switched from checksumming a dummy executable
to checksumming object files). That includes dates (unless built with
-D which we don't do).
Then later we switched to do thin archives so for example libbackend.a
we checksum doesn't contain the actual code anymore...
A pragmatic approach to "fix" things would be to just checksum
gtype-desc.o which should have enough state to cover PCH dependences
if I understand the workings correctly (patch below - a single
checksum would suffice so more simplifications are possible).
Another solution working on ELF systems with build-id support is
simply forgo checksumming anything and rely on the executable
build-id instead (pat^whack below as well).
Does anybody think that just checksumming gtype-desc.o is a
degradation over the current state (which checksums thin archives)?
Thanks,
Richard.
2019-02-22 Richard Biener <rguenther@suse.de>
c/
* Make-lang.in (cc1-checksum.c): Checksum only gtype-desc.o.
cp/
* Make-lang.in (cc1plus-checksum.c): Checksum only gtype-desc.o.
objc/
* Make-lang.in (cc1obj-checksum.c): Checksum only gtype-desc.o.
objcp/
* Make-lang.in (cc1objplus-checksum.c): Checksum only gtype-desc.o.
Index: gcc/c/Make-lang.in
===================================================================
--- gcc/c/Make-lang.in (revision 269111)
+++ gcc/c/Make-lang.in (working copy)
@@ -70,14 +70,13 @@ endif
# compute checksum over all object files and the options
# re-use the checksum from the prev-final stage so it passes
# the bootstrap comparison and allows comparing of the cc1 binary
-cc1-checksum.c : build/genchecksum$(build_exeext) checksum-options \
- $(C_OBJS) $(BACKEND) $(LIBDEPS)
+cc1-checksum.c : build/genchecksum$(build_exeext) gtype-desc.o
if [ -f ../stage_final ] \
&& cmp -s ../stage_current ../stage_final; then \
cp ../prev-gcc/cc1-checksum.c cc1-checksum.c; \
else \
- build/genchecksum$(build_exeext) $(C_OBJS) $(BACKEND) $(LIBDEPS) \
- checksum-options > cc1-checksum.c.tmp && \
+ build/genchecksum$(build_exeext) gtype-desc.o \
+ > cc1-checksum.c.tmp && \
$(srcdir)/../move-if-change cc1-checksum.c.tmp cc1-checksum.c; \
fi
Index: gcc/cp/Make-lang.in
===================================================================
--- gcc/cp/Make-lang.in (revision 269111)
+++ gcc/cp/Make-lang.in (working copy)
@@ -105,14 +105,13 @@ cp-warn = $(STRICT_WARN)
# compute checksum over all object files and the options
# re-use the checksum from the prev-final stage so it passes
# the bootstrap comparison and allows comparing of the cc1 binary
-cc1plus-checksum.c : build/genchecksum$(build_exeext) checksum-options \
- $(CXX_OBJS) $(BACKEND) $(LIBDEPS)
+cc1plus-checksum.c : build/genchecksum$(build_exeext) gtype-desc.o
if [ -f ../stage_final ] \
&& cmp -s ../stage_current ../stage_final; then \
cp ../prev-gcc/cc1plus-checksum.c cc1plus-checksum.c; \
else \
- build/genchecksum$(build_exeext) $(CXX_OBJS) $(BACKEND) $(LIBDEPS) \
- checksum-options > cc1plus-checksum.c.tmp && \
+ build/genchecksum$(build_exeext) gtype-desc.o \
+ > cc1plus-checksum.c.tmp && \
$(srcdir)/../move-if-change cc1plus-checksum.c.tmp cc1plus-checksum.c; \
fi
Index: gcc/objc/Make-lang.in
===================================================================
--- gcc/objc/Make-lang.in (revision 269111)
+++ gcc/objc/Make-lang.in (working copy)
@@ -56,10 +56,9 @@ OBJC_OBJS = objc/objc-lang.o objc/objc-a
objc_OBJS = $(OBJC_OBJS) cc1obj-checksum.o
-cc1obj-checksum.c : build/genchecksum$(build_exeext) checksum-options \
- $(OBJC_OBJS) $(C_AND_OBJC_OBJS) $(BACKEND) $(LIBDEPS)
- build/genchecksum$(build_exeext) $(OBJC_OBJS) $(C_AND_OBJC_OBJS) \
- $(BACKEND) $(LIBDEPS) checksum-options > cc1obj-checksum.c.tmp && \
+cc1obj-checksum.c : build/genchecksum$(build_exeext) gtype-desc.o
+ build/genchecksum$(build_exeext) gtype-desc.o
+ > cc1obj-checksum.c.tmp && \
$(srcdir)/../move-if-change cc1obj-checksum.c.tmp cc1obj-checksum.c
cc1obj$(exeext): $(OBJC_OBJS) $(C_AND_OBJC_OBJS) cc1obj-checksum.o $(BACKEND) $(LIBDEPS)
Index: gcc/objcp/Make-lang.in
===================================================================
--- gcc/objcp/Make-lang.in (revision 269111)
+++ gcc/objcp/Make-lang.in (working copy)
@@ -59,10 +59,9 @@ OBJCXX_OBJS = objcp/objcp-act.o objcp/ob
obj-c++_OBJS = $(OBJCXX_OBJS) cc1objplus-checksum.o
-cc1objplus-checksum.c : build/genchecksum$(build_exeext) checksum-options \
- $(OBJCXX_OBJS) $(BACKEND) $(LIBDEPS)
- build/genchecksum$(build_exeext) $(OBJCXX_OBJS) $(BACKEND) \
- $(LIBDEPS) checksum-options > cc1objplus-checksum.c.tmp && \
+cc1objplus-checksum.c : build/genchecksum$(build_exeext) gtype-desc.o
+ build/genchecksum$(build_exeext) gtype-desc.o
+ > cc1objplus-checksum.c.tmp && \
$(srcdir)/../move-if-change cc1objplus-checksum.c.tmp \
cc1objplus-checksum.c
2019-02-22 Richard Biener <rguenther@suse.de>
* ...
Index: gcc/c-family/c-pch.c
===================================================================
--- gcc/c-family/c-pch.c (revision 269111)
+++ gcc/c-family/c-pch.c (working copy)
@@ -45,10 +45,6 @@ enum {
MATCH_SIZE = ARRAY_SIZE (pch_matching)
};
-/* The value of the checksum in the dummy compiler that is actually
- checksummed. That compiler should never be run. */
-static const char no_checksum[16] = { 0 };
-
/* Information about flags and suchlike that affect PCH validity.
Before this structure is read, both an initial 8-character identification
@@ -69,6 +65,67 @@ static FILE *pch_outfile;
static const char *get_ident (void);
+#if _GNU_SOURCE
+#include <link.h>
+
+#define ALIGN(val, align) (((val) + (align) - 1) & ~((align) - 1))
+
+struct build_id_note {
+ /* The NHdr. */
+ uint32_t namesz;
+ uint32_t descsz;
+ uint32_t type;
+
+ char name[4]; /* Note name for build-id is "GNU\0" */
+ unsigned char build_id[16];
+};
+
+static int
+get_build_id_1 (struct dl_phdr_info *info, size_t, void *data)
+{
+ for (unsigned i = 0; i < info->dlpi_phnum; ++i)
+ {
+ if (info->dlpi_phdr[i].p_type != PT_NOTE)
+ continue;
+ build_id_note *note
+ = (build_id_note *)(info->dlpi_addr + info->dlpi_phdr[i].p_vaddr);
+ ptrdiff_t size = info->dlpi_phdr[i].p_filesz;
+ while (size >= (ptrdiff_t)sizeof (build_id_note))
+ {
+ if (note->type == NT_GNU_BUILD_ID
+ && note->namesz == 4
+ && note->descsz >= 16)
+ {
+ memcpy (data, note->build_id, 16);
+ return 1;
+ }
+ size_t offset = (sizeof (uint32_t) * 3
+ + ALIGN(note->namesz, 4)
+ + ALIGN(note->descsz, 4));
+ note = (build_id_note *)((char *)note + offset);
+ size -= offset;
+ }
+ }
+
+ return 0;
+}
+
+static const unsigned char *
+get_build_id ()
+{
+ static unsigned char build_id[16];
+ if (!dl_iterate_phdr (get_build_id_1, build_id))
+ return NULL;
+ return build_id;
+}
+#else
+static const unsigned char *
+get_build_id ()
+{
+ return NULL;
+}
+#endif
+
/* Compute an appropriate 8-byte magic number for the PCH file, so that
utilities like file(1) can identify it, and so that GCC can quickly
ignore non-PCH files and PCH files that are of a completely different
@@ -111,8 +168,6 @@ pch_init (void)
pch_file);
pch_outfile = f;
- gcc_assert (memcmp (executable_checksum, no_checksum, 16) != 0);
-
memset (&v, '\0', sizeof (v));
v.debug_info_type = write_symbols;
{
@@ -126,8 +181,11 @@ pch_init (void)
v.pch_init = &pch_init;
target_validity = targetm.get_pch_validity (&v.target_data_length);
+ const unsigned char *chksum = get_build_id ();
+ if (!chksum)
+ chksum = executable_checksum;
if (fwrite (partial_pch, IDENT_LENGTH, 1, f) != 1
- || fwrite (executable_checksum, 16, 1, f) != 1
+ || fwrite (chksum, 16, 1, f) != 1
|| fwrite (&v, sizeof (v), 1, f) != 1
|| fwrite (target_validity, v.target_data_length, 1, f) != 1)
fatal_error (input_location, "can%'t write to %s: %m", pch_file);
@@ -212,8 +270,6 @@ c_common_valid_pch (cpp_reader *pfile, c
/* Perform a quick test of whether this is a valid
precompiled header for the current language. */
- gcc_assert (memcmp (executable_checksum, no_checksum, 16) != 0);
-
sizeread = read (fd, ident, IDENT_LENGTH + 16);
if (sizeread == -1)
fatal_error (input_location, "can%'t read %s: %m", name);
@@ -245,7 +301,10 @@ c_common_valid_pch (cpp_reader *pfile, c
}
return 2;
}
- if (memcmp (ident + IDENT_LENGTH, executable_checksum, 16) != 0)
+ const unsigned char *chksum = get_build_id ();
+ if (!chksum)
+ chksum = executable_checksum;
+ if (memcmp (ident + IDENT_LENGTH, chksum, 16) != 0)
{
if (cpp_get_options (pfile)->warn_invalid_pch)
cpp_error (pfile, CPP_DL_WARNING,
Index: gcc/genchecksum.c
===================================================================
--- gcc/genchecksum.c (revision 269111)
+++ gcc/genchecksum.c (working copy)
@@ -113,8 +113,13 @@ main (int argc, char ** argv)
puts ("#include \"config.h\"");
puts ("#include \"system.h\"");
fputs ("EXPORTED_CONST unsigned char executable_checksum[16] = { ", stdout);
+#if _GNU_SOURCE
+ for (i = 0; i < 16; i++)
+ printf ("0x%02x%s", 0, i == 15 ? " };\n" : ", ");
+#else
for (i = 0; i < 16; i++)
printf ("0x%02x%s", result[i], i == 15 ? " };\n" : ", ");
+#endif
return 0;
}
More information about the Gcc
mailing list