x86/vdso: Remove some redundant in-memory section headers

.data doesn't need to be separate from .rodata: they're both readonly.

.altinstructions and .altinstr_replacement aren't needed by anything
except vdso2c; strip them from the final image.

While we're at it, rather than aligning the actual executable text,
just shove some unused-at-runtime data in between real data and
text.

My vdso image is still above 4k, but I'm disinclined to try to
trim it harder for 3.16.  For future trimming, I suspect that these
sections could be moved to later in the file and dropped from
the in-memory image:

.gnu.version and .gnu.version_d   (this may lose versions in gdb)
.eh_frame                         (should be harmless)
.eh_frame_hdr                     (I'm not really sure)
.hash                             (AFAIK nothing needs this section header)

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/2e96d0c49016ea6d026a614ae645e93edd325961.1403129369.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

3 files changed, 26 insertions, 24 deletions

diff --git a/arch/x86/vdso/vdso-fakesections.c b/arch/x86/vdso/vdso-fakesections.c
index 56927a7e4977..aa5fbfab20a5 100644
--- a/arch/x86/vdso/vdso-fakesections.c
+++ b/arch/x86/vdso/vdso-fakesections.c
@@ -16,9 +16,6 @@ const char fake_shstrtab[] __attribute__((section(".fake_shstrtab"))) =
         ".rodata\0"
         ".fake_shstrtab\0"  /* Yay, self-referential code. */
         ".note\0"
-        ".data\0"
-        ".altinstructions\0"
-        ".altinstr_replacement\0"
         ".eh_frame_hdr\0"
         ".eh_frame\0"
         ".text";
diff --git a/arch/x86/vdso/vdso-layout.lds.S b/arch/x86/vdso/vdso-layout.lds.S
index e4cbc2145bab..9197544eea9a 100644
--- a/arch/x86/vdso/vdso-layout.lds.S
+++ b/arch/x86/vdso/vdso-layout.lds.S
@@ -14,7 +14,7 @@
 # error unknown VDSO target
 #endif
 
-#define NUM_FAKE_SHDRS 16
+#define NUM_FAKE_SHDRS 13
 
 SECTIONS
 {
@@ -28,15 +28,17 @@ SECTIONS
         .gnu.version_d  : { *(.gnu.version_d) }
         .gnu.version_r  : { *(.gnu.version_r) }
 
-        .note           : { *(.note.*) }                :text   :note
-
-        .eh_frame_hdr   : { *(.eh_frame_hdr) }          :text   :eh_frame_hdr
-        .eh_frame       : { KEEP (*(.eh_frame)) }       :text
-
         .dynamic        : { *(.dynamic) }               :text   :dynamic
 
         .rodata         : {
                 *(.rodata*)
+                *(.data*)
+                *(.sdata*)
+                *(.got.plt) *(.got)
+                *(.gnu.linkonce.d.*)
+                *(.bss*)
+                *(.dynbss*)
+                *(.gnu.linkonce.b.*)
 
                 /*
                  * Ideally this would live in a C file, but that won't
@@ -50,28 +52,29 @@ SECTIONS
 
         .fake_shstrtab  : { *(.fake_shstrtab) }         :text
 
-        .data           : {
-                *(.data*)
-                *(.sdata*)
-                *(.got.plt) *(.got)
-                *(.gnu.linkonce.d.*)
-                *(.bss*)
-                *(.dynbss*)
-                *(.gnu.linkonce.b.*)
-        }
 
-        .altinstructions        : { *(.altinstructions) }
-        .altinstr_replacement   : { *(.altinstr_replacement) }
+        .note           : { *(.note.*) }                :text   :note
+
+        .eh_frame_hdr   : { *(.eh_frame_hdr) }          :text   :eh_frame_hdr
+        .eh_frame       : { KEEP (*(.eh_frame)) }       :text
+
 
         /*
-         * Align the actual code well away from the non-instruction data.
-         * This is the best thing for the I-cache.
+         * Text is well-separated from actual data: there's plenty of
+         * stuff that isn't used at runtime in between.
          */
-        . = ALIGN(0x100);
 
         .text           : { *(.text*) }                 :text   =0x90909090,
 
         /*
+         * At the end so that eu-elflint stays happy when vdso2c strips
+         * these.  A better implementation would avoid allocating space
+         * for these.
+         */
+        .altinstructions        : { *(.altinstructions) }       :text
+        .altinstr_replacement   : { *(.altinstr_replacement) }  :text
+
+        /*
          * The remainder of the vDSO consists of special pages that are
          * shared between the kernel and userspace.  It needs to be at the
          * end so that it doesn't overlap the mapping of the actual
diff --git a/arch/x86/vdso/vdso2c.h b/arch/x86/vdso/vdso2c.h
index f01ed4bde880..f42e2ddc663d 100644
--- a/arch/x86/vdso/vdso2c.h
+++ b/arch/x86/vdso/vdso2c.h
@@ -92,7 +92,9 @@ static void BITSFUNC(copy_section)(struct BITSFUNC(fake_sections) *out,
 {
         uint64_t flags = GET_LE(&in->sh_flags);
 
-        bool copy = flags & SHF_ALLOC;
+        bool copy = flags & SHF_ALLOC &&
+                strcmp(name, ".altinstructions") &&
+                strcmp(name, ".altinstr_replacement");
 
         if (!copy)
                 return;