x86/vdso: Improve the fake section headers

Fully stripping the vDSO has other unfortunate side effects:

 - binutils is unable to find ELF notes without a SHT_NOTE section.

 - Even elfutils has trouble: it can find ELF notes without a section
   table at all, but if a section table is present, it won't look for
   PT_NOTE.

 - gdb wants section names to match between stripped DSOs and their
   symbols; otherwise it will corrupt symbol addresses.

We're also breaking the rules: section 0 is supposed to be SHT_NULL.

Fix these problems by building a better fake section table.  While
we're at it, we might as well let buggy Go versions keep working well
by giving the SHT_DYNSYM entry the correct size.

This is a bit unfortunate: it adds quite a bit of size to the vdso
image.

If/when binutils improves and the improved versions become widespread,
it would be worth considering dropping most of this.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/0e546a5eeaafdf1840e6ee654a55c1e727c26663.1403129369.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 158 insertions, 22 deletions

diff --git a/arch/x86/vdso/vdso2c.h b/arch/x86/vdso/vdso2c.h
index 8e185ce39e69..f01ed4bde880 100644
--- a/arch/x86/vdso/vdso2c.h
+++ b/arch/x86/vdso/vdso2c.h
@@ -4,6 +4,116 @@
  * are built for 32-bit userspace.
  */
 
+/*
+ * We're writing a section table for a few reasons:
+ *
+ * The Go runtime had a couple of bugs: it would read the section
+ * table to try to figure out how many dynamic symbols there were (it
+ * shouldn't have looked at the section table at all) and, if there
+ * were no SHT_SYNDYM section table entry, it would use an
+ * uninitialized value for the number of symbols.  An empty DYNSYM
+ * table would work, but I see no reason not to write a valid one (and
+ * keep full performance for old Go programs).  This hack is only
+ * needed on x86_64.
+ *
+ * The bug was introduced on 2012-08-31 by:
+ * https://code.google.com/p/go/source/detail?r=56ea40aac72b
+ * and was fixed on 2014-06-13 by:
+ * https://code.google.com/p/go/source/detail?r=fc1cd5e12595
+ *
+ * Binutils has issues debugging the vDSO: it reads the section table to
+ * find SHT_NOTE; it won't look at PT_NOTE for the in-memory vDSO, which
+ * would break build-id if we removed the section table.  Binutils
+ * also requires that shstrndx != 0.  See:
+ * https://sourceware.org/bugzilla/show_bug.cgi?id=17064
+ *
+ * elfutils might not look for PT_NOTE if there is a section table at
+ * all.  I don't know whether this matters for any practical purpose.
+ *
+ * For simplicity, rather than hacking up a partial section table, we
+ * just write a mostly complete one.  We omit non-dynamic symbols,
+ * though, since they're rather large.
+ *
+ * Once binutils gets fixed, we might be able to drop this for all but
+ * the 64-bit vdso, since build-id only works in kernel RPMs, and
+ * systems that update to new enough kernel RPMs will likely update
+ * binutils in sync.  build-id has never worked for home-built kernel
+ * RPMs without manual symlinking, and I suspect that no one ever does
+ * that.
+ */
+struct BITSFUNC(fake_sections)
+{
+        ELF(Shdr) *table;
+        unsigned long table_offset;
+        int count, max_count;
+
+        int in_shstrndx;
+        unsigned long shstr_offset;
+        const char *shstrtab;
+        size_t shstrtab_len;
+
+        int out_shstrndx;
+};
+
+static unsigned int BITSFUNC(find_shname)(struct BITSFUNC(fake_sections) *out,
+                                          const char *name)
+{
+        const char *outname = out->shstrtab;
+        while (outname - out->shstrtab < out->shstrtab_len) {
+                if (!strcmp(name, outname))
+                        return (outname - out->shstrtab) + out->shstr_offset;
+                outname += strlen(outname) + 1;
+        }
+
+        if (*name)
+                printf("Warning: could not find output name \"%s\"\n", name);
+        return out->shstr_offset + out->shstrtab_len - 1;  /* Use a null. */
+}
+
+static void BITSFUNC(init_sections)(struct BITSFUNC(fake_sections) *out)
+{
+        if (!out->in_shstrndx)
+                fail("didn't find the fake shstrndx\n");
+
+        memset(out->table, 0, out->max_count * sizeof(ELF(Shdr)));
+
+        if (out->max_count < 1)
+                fail("we need at least two fake output sections\n");
+
+        PUT_LE(&out->table[0].sh_type, SHT_NULL);
+        PUT_LE(&out->table[0].sh_name, BITSFUNC(find_shname)(out, ""));
+
+        out->count = 1;
+}
+
+static void BITSFUNC(copy_section)(struct BITSFUNC(fake_sections) *out,
+                                   int in_idx, const ELF(Shdr) *in,
+                                   const char *name)
+{
+        uint64_t flags = GET_LE(&in->sh_flags);
+
+        bool copy = flags & SHF_ALLOC;
+
+        if (!copy)
+                return;
+
+        if (out->count >= out->max_count)
+                fail("too many copied sections (max = %d)\n", out->max_count);
+
+        if (in_idx == out->in_shstrndx)
+                out->out_shstrndx = out->count;
+
+        out->table[out->count] = *in;
+        PUT_LE(&out->table[out->count].sh_name,
+               BITSFUNC(find_shname)(out, name));
+
+        /* elfutils requires that a strtab have the correct type. */
+        if (!strcmp(name, ".fake_shstrtab"))
+                PUT_LE(&out->table[out->count].sh_type, SHT_STRTAB);
+
+        out->count++;
+}
+
 static void BITSFUNC(go)(void *addr, size_t len,
                          FILE *outfile, const char *name)
 {
@@ -19,7 +129,7 @@ static void BITSFUNC(go)(void *addr, size_t len,
         const char *secstrings;
         uint64_t syms[NSYMS] = {};
 
-        uint64_t fake_sections_value = 0, fake_sections_size = 0;
+        struct BITSFUNC(fake_sections) fake_sections = {};
 
         ELF(Phdr) *pt = (ELF(Phdr) *)(addr + GET_LE(&hdr->e_phoff));
 
@@ -89,23 +199,57 @@ static void BITSFUNC(go)(void *addr, size_t len,
                         GET_LE(&sym->st_name);
 
                 for (k = 0; k < NSYMS; k++) {
-                        if (!strcmp(name, required_syms[k])) {
+                        if (!strcmp(name, required_syms[k].name)) {
                                 if (syms[k]) {
                                         fail("duplicate symbol %s\n",
-                                             required_syms[k]);
+                                             required_syms[k].name);
                                 }
                                 syms[k] = GET_LE(&sym->st_value);
                         }
                 }
 
-                if (!strcmp(name, "vdso_fake_sections")) {
-                        if (fake_sections_value)
-                                fail("duplicate vdso_fake_sections\n");
-                        fake_sections_value = GET_LE(&sym->st_value);
-                        fake_sections_size = GET_LE(&sym->st_size);
+                if (!strcmp(name, "fake_shstrtab")) {
+                        ELF(Shdr) *sh;
+
+                        fake_sections.in_shstrndx = GET_LE(&sym->st_shndx);
+                        fake_sections.shstrtab = addr + GET_LE(&sym->st_value);
+                        fake_sections.shstrtab_len = GET_LE(&sym->st_size);
+                        sh = addr + GET_LE(&hdr->e_shoff) +
+                                GET_LE(&hdr->e_shentsize) *
+                                fake_sections.in_shstrndx;
+                        fake_sections.shstr_offset = GET_LE(&sym->st_value) -
+                                GET_LE(&sh->sh_addr);
                 }
         }
 
+        /* Build the output section table. */
+        if (!syms[sym_VDSO_FAKE_SECTION_TABLE_START] ||
+            !syms[sym_VDSO_FAKE_SECTION_TABLE_END])
+                fail("couldn't find fake section table\n");
+        if ((syms[sym_VDSO_FAKE_SECTION_TABLE_END] -
+             syms[sym_VDSO_FAKE_SECTION_TABLE_START]) % sizeof(ELF(Shdr)))
+                fail("fake section table size isn't a multiple of sizeof(Shdr)\n");
+        fake_sections.table = addr + syms[sym_VDSO_FAKE_SECTION_TABLE_START];
+        fake_sections.table_offset = syms[sym_VDSO_FAKE_SECTION_TABLE_START];
+        fake_sections.max_count = (syms[sym_VDSO_FAKE_SECTION_TABLE_END] -
+                                   syms[sym_VDSO_FAKE_SECTION_TABLE_START]) /
+                sizeof(ELF(Shdr));
+
+        BITSFUNC(init_sections)(&fake_sections);
+        for (i = 0; i < GET_LE(&hdr->e_shnum); i++) {
+                ELF(Shdr) *sh = addr + GET_LE(&hdr->e_shoff) +
+                        GET_LE(&hdr->e_shentsize) * i;
+                BITSFUNC(copy_section)(&fake_sections, i, sh,
+                                       secstrings + GET_LE(&sh->sh_name));
+        }
+        if (!fake_sections.out_shstrndx)
+                fail("didn't generate shstrndx?!?\n");
+
+        PUT_LE(&hdr->e_shoff, fake_sections.table_offset);
+        PUT_LE(&hdr->e_shentsize, sizeof(ELF(Shdr)));
+        PUT_LE(&hdr->e_shnum, fake_sections.count);
+        PUT_LE(&hdr->e_shstrndx, fake_sections.out_shstrndx);
+
         /* Validate mapping addresses. */
         for (i = 0; i < sizeof(special_pages) / sizeof(special_pages[0]); i++) {
                 if (!syms[i])
@@ -113,25 +257,17 @@ static void BITSFUNC(go)(void *addr, size_t len,
 
                 if (syms[i] % 4096)
                         fail("%s must be a multiple of 4096\n",
-                             required_syms[i]);
+                             required_syms[i].name);
                 if (syms[i] < data_size)
                         fail("%s must be after the text mapping\n",
-                             required_syms[i]);
+                             required_syms[i].name);
                 if (syms[sym_end_mapping] < syms[i] + 4096)
-                        fail("%s overruns end_mapping\n", required_syms[i]);
+                        fail("%s overruns end_mapping\n",
+                             required_syms[i].name);
         }
         if (syms[sym_end_mapping] % 4096)
                 fail("end_mapping must be a multiple of 4096\n");
 
-        /* Remove sections or use fakes */
-        if (fake_sections_size % sizeof(ELF(Shdr)))
-                fail("vdso_fake_sections size is not a multiple of %ld\n",
-                     (long)sizeof(ELF(Shdr)));
-        PUT_LE(&hdr->e_shoff, fake_sections_value);
-        PUT_LE(&hdr->e_shentsize, fake_sections_value ? sizeof(ELF(Shdr)) : 0);
-        PUT_LE(&hdr->e_shnum, fake_sections_size / sizeof(ELF(Shdr)));
-        PUT_LE(&hdr->e_shstrndx, SHN_UNDEF);
-
         if (!name) {
                 fwrite(addr, load_size, 1, outfile);
                 return;
@@ -169,9 +305,9 @@ static void BITSFUNC(go)(void *addr, size_t len,
                         (unsigned long)GET_LE(&alt_sec->sh_size));
         }
         for (i = 0; i < NSYMS; i++) {
-                if (syms[i])
+                if (required_syms[i].export && syms[i])
                         fprintf(outfile, "\t.sym_%s = 0x%" PRIx64 ",\n",
-                                required_syms[i], syms[i]);
+                                required_syms[i].name, syms[i]);
         }
         fprintf(outfile, "};\n");
 }