[PATCH v2 0/3] powerpc: Add to linker script discards

v2 pulls in the branch_lt patch too. No changes to the first two
patches.

Joel Stanley (3):
  powerpc: Discard more sections in linker script
  powerpc: Discard dynsym section for !PPC32
  powerpc: Discard .branch_lt section

 arch/powerpc/kernel/vmlinux.lds.S | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

-- 
2.19.1

Building the ppc64 kernel with a modern binutils results in this
warning:

 powerpc64le-linux-gnu-ld: warning: orphan section `.gnu.hash' from
 `linker stubs' being placed in section `.gnu.hash'

Alan Modra [off-list ref] explains:

 > .gnu.hash, like .hash, is used by glibc ld.so for dynamic symbol
 > lookup.  I imagine you don't need either section in a kernel, so
 > discarding both sounds reasonable.  Likely you could discard .interp
 > and .dynstr too, and .dynsym when !CONFIG_PPC32.

Reported-by: Stephen Rothwell <redacted>
Signed-off-by: Joel Stanley <joel@jms.id.au>
---
See https://lore.kernel.org/lkml/CACPK8Xft3n5KkpTjN3=7_VUCXHFcK7mxvZm2Rrqu7tppcBoyOg@mail.gmail.com/T/#m58532c86cf0c7b4fb01cc1fe724e48d4c7d8e4a7
---
 arch/powerpc/kernel/vmlinux.lds.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 434581bcd5b4..6570209b0671 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S

@@ -279,8 +279,6 @@ SECTIONS
 		__dynamic_start = .;
 		*(.dynamic)
 	}
-	.hash : AT(ADDR(.hash) - LOAD_OFFSET) { *(.hash) }
-	.interp : AT(ADDR(.interp) - LOAD_OFFSET) { *(.interp) }
 	.rela.dyn : AT(ADDR(.rela.dyn) - LOAD_OFFSET)
 	{
 		__rela_dyn_start = .;

@@ -388,5 +386,8 @@ SECTIONS
 		*(.gnu.version*)
 		*(.gnu.attributes)
 		*(.eh_frame)
+		*(.hash .gnu.hash)
+		*(.interp)
+		*(.dynstr)
 	}
 }

-- 
2.19.1

Alan Modra [off-list ref] explains:

 > Likely you could discard .interp > and .dynstr too, and .dynsym when
 > !CONFIG_PPC32.

Discarding of interp and dynstr happened in a previous patch. The dynsym
cleanup was a bit less straightforward, so it gets it's own patch.

Signed-off-by: Joel Stanley <joel@jms.id.au>
---
See https://lore.kernel.org/lkml/CACPK8Xft3n5KkpTjN3=7_VUCXHFcK7mxvZm2Rrqu7tppcBoyOg@mail.gmail.com/T/#m58532c86cf0c7b4fb01cc1fe724e48d4c7d8e4a7
---
 arch/powerpc/kernel/vmlinux.lds.S | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 6570209b0671..2c93a420f456 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S

@@ -266,14 +266,13 @@ SECTIONS
 	}
 #ifdef CONFIG_RELOCATABLE
 	. = ALIGN(8);
+#ifdef CONFIG_PPC32
 	.dynsym : AT(ADDR(.dynsym) - LOAD_OFFSET)
 	{
-#ifdef CONFIG_PPC32
 		__dynamic_symtab = .;
-#endif
 		*(.dynsym)
 	}
-	.dynstr : AT(ADDR(.dynstr) - LOAD_OFFSET) { *(.dynstr) }
+#endif
 	.dynamic : AT(ADDR(.dynamic) - LOAD_OFFSET)
 	{
 		__dynamic_start = .;

@@ -389,5 +388,8 @@ SECTIONS
 		*(.hash .gnu.hash)
 		*(.interp)
 		*(.dynstr)
+#ifndef CONFIG_PPC32
+		*(.dynsym)
+#endif
 	}
 }

-- 
2.19.1

When building a 32 bit powerpc kernel with Binutils 2.31.1 this warning
is emitted:

 powerpc-linux-gnu-ld: warning: orphan section `.branch_lt' from
 `arch/powerpc/kernel/head_44x.o' being placed in section `.branch_lt'

As of binutils commit 2d7ad24e8726 ("Support PLT16 relocs against local
symbols")[1], 32 bit targets can produce .branch_lt sections in their
output.

These sections should be empty for the kernel build so discard them for
both PPC64 and PPC32.

[1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=2d7ad24e8726ba4c45c9e67be08223a146a837ce
Signed-off-by: Joel Stanley <joel@jms.id.au>
---
v2: Discard instead of keep. Alan said "[discarding] can be a recipe for
finding linker bugs", so lets go with that.
---
 arch/powerpc/kernel/vmlinux.lds.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 2c93a420f456..af0f81df8bb9 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S

@@ -316,7 +316,6 @@ SECTIONS
 		DATA_DATA
 		*(.data.rel*)
 		*(.toc1)
-		*(.branch_lt)
 	}
 
 	.opd : AT(ADDR(.opd) - LOAD_OFFSET) {

@@ -388,6 +387,7 @@ SECTIONS
 		*(.hash .gnu.hash)
 		*(.interp)
 		*(.dynstr)
+		*(.branch_lt)
 #ifndef CONFIG_PPC32
 		*(.dynsym)
 #endif

-- 
2.19.1

On Tue, Dec 04, 2018 at 11:24:28AM +1030, Joel Stanley wrote:

Alan Modra [off-list ref] explains:

 > Likely you could discard .interp > and .dynstr too, and .dynsym when
 > !CONFIG_PPC32.

Discarding of interp and dynstr happened in a previous patch. The dynsym
cleanup was a bit less straightforward, so it gets it's own patch.

So this last line belongs in the previous patch then, right?


Segher

On Wed, 5 Dec 2018 at 04:11, Segher Boessenkool
[off-list ref] wrote:

On Tue, Dec 04, 2018 at 11:24:28AM +1030, Joel Stanley wrote:

quoted

Alan Modra [off-list ref] explains:

 > Likely you could discard .interp > and .dynstr too, and .dynsym when
 > !CONFIG_PPC32.

Discarding of interp and dynstr happened in a previous patch. The dynsym
cleanup was a bit less straightforward, so it gets it's own patch.

quoted

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 6570209b0671..2c93a420f456 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S

@@ -266,14 +266,13 @@ SECTIONS
      }
 #ifdef CONFIG_RELOCATABLE
      . = ALIGN(8);
+#ifdef CONFIG_PPC32
      .dynsym : AT(ADDR(.dynsym) - LOAD_OFFSET)
      {
-#ifdef CONFIG_PPC32
              __dynamic_symtab = .;
-#endif
              *(.dynsym)
      }
-     .dynstr : AT(ADDR(.dynstr) - LOAD_OFFSET) { *(.dynstr) }

So this last line belongs in the previous patch then, right?

Correct. Good catch.