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>

diff --git a/arch/x86/vdso/vdso-fakesections.c b/arch/x86/vdso/vdso-fakesections.c
index 56927a7..aa5fbfa 100644 (file)
--- 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 e4cbc21..9197544 100644 (file)
--- 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
@@ -50,27 +52,28 @@ 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

diff --git a/arch/x86/vdso/vdso2c.h b/arch/x86/vdso/vdso2c.h
index f01ed4b..f42e2dd 100644 (file)
--- 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;