[PATCH v2] configure: Use lld --image-base for --disable-pie user mode binaries

[PATCH v2] configure: Use lld --image-base for --disable-pie user mode binaries

[Fangrui Song]

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On 2019-11-27, Alex Bennée wrote:
>
>Fangrui Song <i@maskray.me> writes:
>
>> For lld, --image-base is the preferred way to set the base address.
>> lld does not actually implement -Ttext-segment, but treats it as an alias for
>> -Ttext. -Ttext-segment=0x60000000 combined with --no-rosegment can
>> create a 1.6GB executable.
>>
>> Fix the problem by using --image-base for lld. GNU ld and gold will
>> still get -Ttext-segment. Also delete the ld --verbose fallback introduced
>> in 2013, which is no longer relevant or correct (the default linker
>> script has changed).
>>
>> Signed-off-by: Fangrui Song <i@maskray.me>
>
>This patch no longer applies cleanly to configure so I couldn't test it.

configure was touched twice last week, but git still works.
Anyway, I am uploading a new patch without In-Reply-To.

>> ---
>>   configure | 33 ++++++++++++---------------------
>>   1 file changed, 12 insertions(+), 21 deletions(-)
>>
>> diff --git a/configure b/configure
>> index 6099be1d84..2d45af0d09 100755
>> --- a/configure
>> +++ b/configure
>> @@ -6336,43 +6336,34 @@ fi
>>
>>   # Probe for the need for relocating the user-only binary.
>>   if ( [ "$linux_user" = yes ] || [ "$bsd_user" = yes ] ) && [ "$pie" = no ]; then
>> -  textseg_addr=
>> +  image_base=
>>     case "$cpu" in
>>       arm | i386 | ppc* | s390* | sparc* | x86_64 | x32)
>> -      # ??? Rationale for choosing this address
>> -      textseg_addr=0x60000000
>> +      # An arbitrary address that makes it unlikely to collide with user
>> +      # programs.
>> +      image_base=0x60000000
>
>The comment probably belongs up above when we define the empty variable
>unless it really is specifically about these targets.

Moved the comment.

>Renaming textseg_addr seems like unnecessary churn for this patch.

Renaming is necessary. "Text segment" is no more an appropriate term
even with GNU ld due to the prevalence of -z separate-code
(--enable-separate-code is the default on Linux x86-64 in recent
binutils releases). There is a PF_R PT_LOAD before PF_R|PF_X PT_LOAD
(text segment). One may argue that we can name the two PT_LOAD
collectively as "text segment but I don't think that is appropriate.

>
>>         ;;
>>       mips)
>>         # A 256M aligned address, high in the address space, with enough
>>         # room for the code_gen_buffer above it before the stack.
>> -      textseg_addr=0x60000000
>> +      image_base=0x60000000
>>         ;;
>>     esac
>> -  if [ -n "$textseg_addr" ]; then
>> +  if [ -n "$image_base" ]; then
>>       cat > $TMPC <<EOF
>>       int main(void) { return 0; }
>>   EOF
>> -    textseg_ldflags="-Wl,-Ttext-segment=$textseg_addr"
>> -    if ! compile_prog "" "$textseg_ldflags"; then
>> -      # In case ld does not support -Ttext-segment, edit the default linker
>> -      # script via sed to set the .text start addr.  This is needed on FreeBSD
>> -      # at least.
>> -      if ! $ld --verbose >/dev/null 2>&1; then
>> +    image_base_ldflags="-Wl,--image-base=$image_base"
>> +    if ! compile_prog "" "$image_base_ldflags"; then
>> +      image_base_ldflags="-Wl,-Ttext-segment=$image_base"
>> +      if ! compile_prog "" "$image_base_ldflags"; then
>>           error_exit \
>>               "We need to link the QEMU user mode binaries at a" \
>>               "specific text address. Unfortunately your linker" \
>> -            "doesn't support either the -Ttext-segment option or" \
>> -            "printing the default linker script with --verbose." \
>> +            "supports neither --image-base nor -Ttext-segment. " \
>>               "If you don't want the user mode binaries, pass the" \
>>               "--disable-user option to configure."
>>         fi
>> -
>> -      $ld --verbose | sed \
>> -        -e '1,/==================================================/d' \
>> -        -e '/==================================================/,$d' \
>> -        -e "s/[.] = [0-9a-fx]* [+] SIZEOF_HEADERS/. = $textseg_addr + SIZEOF_HEADERS/" \
>> -        -e "s/__executable_start = [0-9a-fx]*/__executable_start = $textseg_addr/" > config-host.ld
>> -      textseg_ldflags="-Wl,-T../config-host.ld"
>>       fi
>>     fi
>>   fi
>> @@ -7945,7 +7936,7 @@ if test "$gprof" = "yes" ; then
>>   fi
>>
>>   if test "$target_linux_user" = "yes" || test "$target_bsd_user" = "yes" ; then
>> -  ldflags="$ldflags $textseg_ldflags"
>> +  ldflags="$ldflags $image_base_ldflags"
>>   fi
>>
>>   # Newer kernels on s390 check for an S390_PGSTE program header and

(I'm not subscribed to the list, so please CC me.
I made this reply by downloading
https://lists.gnu.org/archive/mbox/qemu-devel/2019-11)

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From fea2e6a09234d13402dd6e6dce9300bc6d08efe1 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i@maskray.me>
Date: Fri, 15 Nov 2019 20:42:40 -0800
Subject: [PATCH] configure: Use lld --image-base for --disable-pie user mode
 binaries
To: qemu-devel@nongnu.org

For lld, --image-base is the preferred way to set the base address.
lld does not actually implement -Ttext-segment, but treats it as an alias for
-Ttext. -Ttext-segment=0x60000000 combined with --no-rosegment can
create a 1.6GB executable.

Fix the problem by using --image-base for lld. GNU ld and gold will
still get -Ttext-segment. Also delete the ld --verbose fallback introduced
in 2013, which is no longer relevant or correct (the default linker
script has changed).
---
 configure | 33 ++++++++++++---------------------
 1 file changed, 12 insertions(+), 21 deletions(-)

diff --git a/configure b/configure
index 6099be1d84..33e5344194 100755
--- a/configure
+++ b/configure
@@ -6336,43 +6336,34 @@ fi
 
 # Probe for the need for relocating the user-only binary.
 if ( [ "$linux_user" = yes ] || [ "$bsd_user" = yes ] ) && [ "$pie" = no ]; then
-  textseg_addr=
+  # An arbitrary address that makes it unlikely to collide with user
+  # programs.
+  image_base=
   case "$cpu" in
     arm | i386 | ppc* | s390* | sparc* | x86_64 | x32)
-      # ??? Rationale for choosing this address
-      textseg_addr=0x60000000
+      image_base=0x60000000
       ;;
     mips)
       # A 256M aligned address, high in the address space, with enough
       # room for the code_gen_buffer above it before the stack.
-      textseg_addr=0x60000000
+      image_base=0x60000000
       ;;
   esac
-  if [ -n "$textseg_addr" ]; then
+  if [ -n "$image_base" ]; then
     cat > $TMPC <<EOF
     int main(void) { return 0; }
 EOF
-    textseg_ldflags="-Wl,-Ttext-segment=$textseg_addr"
-    if ! compile_prog "" "$textseg_ldflags"; then
-      # In case ld does not support -Ttext-segment, edit the default linker
-      # script via sed to set the .text start addr.  This is needed on FreeBSD
-      # at least.
-      if ! $ld --verbose >/dev/null 2>&1; then
+    image_base_ldflags="-Wl,--image-base=$image_base"
+    if ! compile_prog "" "$image_base_ldflags"; then
+      image_base_ldflags="-Wl,-Ttext-segment=$image_base"
+      if ! compile_prog "" "$image_base_ldflags"; then
         error_exit \
             "We need to link the QEMU user mode binaries at a" \
             "specific text address. Unfortunately your linker" \
-            "doesn't support either the -Ttext-segment option or" \
-            "printing the default linker script with --verbose." \
+            "supports neither --image-base nor -Ttext-segment. " \
             "If you don't want the user mode binaries, pass the" \
             "--disable-user option to configure."
       fi
-
-      $ld --verbose | sed \
-        -e '1,/==================================================/d' \
-        -e '/==================================================/,$d' \
-        -e "s/[.] = [0-9a-fx]* [+] SIZEOF_HEADERS/. = $textseg_addr + SIZEOF_HEADERS/" \
-        -e "s/__executable_start = [0-9a-fx]*/__executable_start = $textseg_addr/" > config-host.ld
-      textseg_ldflags="-Wl,-T../config-host.ld"
     fi
   fi
 fi
@@ -7945,7 +7936,7 @@ if test "$gprof" = "yes" ; then
 fi
 
 if test "$target_linux_user" = "yes" || test "$target_bsd_user" = "yes" ; then
-  ldflags="$ldflags $textseg_ldflags"
+  ldflags="$ldflags $image_base_ldflags"
 fi
 
 # Newer kernels on s390 check for an S390_PGSTE program header and
-- 
2.24.0