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Start of relro segment adjustment
- From: Alan Modra <amodra at gmail dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Binutils <binutils at sourceware dot org>
- Date: Wed, 1 Apr 2015 19:41:56 +1030
- Subject: Start of relro segment adjustment
- Authentication-results: sourceware.org; auth=none
- References: <CAMe9rOrVpthhMfZEXcMkKzmHcxymfQHKuTuZPi_y36=s6KJ9iA at mail dot gmail dot com> <20150328041540 dot GE26234 at bubble dot grove dot modra dot org> <CAMe9rOoVzbOvdE1_apgUbHtm0gG89d3_9qbnA8=Y3Qu9MznUPA at mail dot gmail dot com> <CAMe9rOp8sdCbOp0eVzRmwMhe_3c2K0EJ6fmnMMWyA01KYamzLw at mail dot gmail dot com> <20150329034858 dot GH26234 at bubble dot grove dot modra dot org> <CAMe9rOrJB7xNLn4FEq5+N8VbnfZuYxkM4tPNMQzOKapVheGOpQ at mail dot gmail dot com> <20150330123934 dot GJ26234 at bubble dot grove dot modra dot org> <CAMe9rOrXQJmjNR4ynGD9BQJz5T8OygQekoLVsmubpFva1ff45w at mail dot gmail dot com> <20150330141433 dot GL26234 at bubble dot grove dot modra dot org> <CAMe9rOrz-6i8M8W2W8d3T60fsew6NVTsW=JQbKreHVnRwHcrzw at mail dot gmail dot com>
Adjusting the start of the relro segment in order to make it end
exactly on a page boundary runs into difficulties when sections in the
relro segment are aligned; Adjusting the start by (next_page - end)
sometimes results in more than that adjustment occurring at the end,
overrunning the page boundary. So when that occurs we try a new lower
start position by masking the adjusted start with the maximum section
alignment. However, we didn't consider that this masked start address
may in fact be before the initial relro base, which is silly since
that can only increase padding at the relro end.
I've also moved some calculations closer to where they are used, and
comments closer to the relevant statements.
* ldlang.c (lang_size_sections): When alignment of sections
results in relro base adjustment being too large, don't go lower
than the initial value.
* ldexp.c (fold_binary <DATA_SEGMENT_RELRO_END>): Comment.
* scripttempl/elf.sc (DATA_SEGMENT_ALIGN): Omit SEGMENT_SIZE
alignment when SEGMENT_SIZE is the same as MAXPAGESIZE.
diff --git a/ld/ldexp.c b/ld/ldexp.c
index ac66cc0..9cd9e29 100644
--- a/ld/ldexp.c
+++ b/ld/ldexp.c
@@ -588,6 +588,8 @@ fold_binary (etree_type *tree)
break;
case DATA_SEGMENT_RELRO_END:
+ /* Operands swapped! DATA_SEGMENT_RELRO_END(offset,exp)
+ has offset in expld.result and exp in lhs. */
expld.dataseg.relro = exp_dataseg_relro_end;
if (expld.phase == lang_first_phase_enum
|| expld.section != bfd_abs_section_ptr)
diff --git a/ld/ldlang.c b/ld/ldlang.c
index 8880821..13e7b1a 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -5382,20 +5382,20 @@ lang_size_sections (bfd_boolean *relax, bfd_boolean check_regions)
if (expld.dataseg.phase == exp_dataseg_end_seen
&& link_info.relro && expld.dataseg.relro_end)
{
- /* If DATA_SEGMENT_ALIGN DATA_SEGMENT_RELRO_END pair was seen, try
- to put expld.dataseg.relro_end on a (common) page boundary. */
- bfd_vma min_base, relro_end, maxpage;
+ bfd_vma initial_base, min_base, relro_end, maxpage;
expld.dataseg.phase = exp_dataseg_relro_adjust;
maxpage = expld.dataseg.maxpagesize;
- /* MIN_BASE is the absolute minimum address we are allowed to start the
- read-write segment (byte before will be mapped read-only). */
- min_base = (expld.dataseg.min_base + maxpage - 1) & ~(maxpage - 1);
+ initial_base = expld.dataseg.base;
+ /* Try to put expld.dataseg.relro_end on a (common) page boundary. */
expld.dataseg.base += (-expld.dataseg.relro_end
& (expld.dataseg.pagesize - 1));
/* Compute the expected PT_GNU_RELRO segment end. */
relro_end = ((expld.dataseg.relro_end + expld.dataseg.pagesize - 1)
& ~(expld.dataseg.pagesize - 1));
+ /* MIN_BASE is the absolute minimum address we are allowed to start the
+ read-write segment (byte before will be mapped read-only). */
+ min_base = (expld.dataseg.min_base + maxpage - 1) & ~(maxpage - 1);
if (min_base + maxpage < expld.dataseg.base)
{
expld.dataseg.base -= maxpage;
@@ -5420,16 +5420,17 @@ lang_size_sections (bfd_boolean *relax, bfd_boolean check_regions)
&& sec->alignment_power > max_alignment_power)
max_alignment_power = sec->alignment_power;
- if (((bfd_vma) 1 << max_alignment_power) < expld.dataseg.pagesize)
- {
- /* Aligning the adjusted base guarantees the padding
- between sections won't change. This is better than
- simply subtracting 1 << max_alignment_power which is
- what we used to do here. */
- expld.dataseg.base &= ~((1 << max_alignment_power) - 1);
- lang_reset_memory_regions ();
- one_lang_size_sections_pass (relax, check_regions);
- }
+ /* Aligning the adjusted base guarantees the padding
+ between sections won't change. This is better than
+ simply subtracting 1 << max_alignment_power which is
+ what we used to do here. */
+ expld.dataseg.base &= ~((1 << max_alignment_power) - 1);
+ /* It doesn't make much sense to go lower than the initial
+ base. That can only increase padding. */
+ if (expld.dataseg.base < initial_base)
+ expld.dataseg.base = initial_base;
+ lang_reset_memory_regions ();
+ one_lang_size_sections_pass (relax, check_regions);
}
link_info.relro_start = expld.dataseg.base;
link_info.relro_end = expld.dataseg.relro_end;
diff --git a/ld/scripttempl/elf.sc b/ld/scripttempl/elf.sc
index c9c80b0..b4f52eb 100644
--- a/ld/scripttempl/elf.sc
+++ b/ld/scripttempl/elf.sc
@@ -128,7 +128,11 @@ DATA_SEGMENT_ALIGN="ALIGN(${SEGMENT_SIZE}) + (. & (${MAXPAGESIZE} - 1))"
DATA_SEGMENT_RELRO_END=""
DATA_SEGMENT_END=""
if test -n "${COMMONPAGESIZE}"; then
- DATA_SEGMENT_ALIGN="ALIGN (${SEGMENT_SIZE}) - ((${MAXPAGESIZE} - .) & (${MAXPAGESIZE} - 1)); . = DATA_SEGMENT_ALIGN (${MAXPAGESIZE}, ${COMMONPAGESIZE})"
+ if test "${SEGMENT_SIZE}" != "${MAXPAGESIZE}"; then
+ DATA_SEGMENT_ALIGN="ALIGN (${SEGMENT_SIZE}) - ((${MAXPAGESIZE} - .) & (${MAXPAGESIZE} - 1)); . = DATA_SEGMENT_ALIGN (${MAXPAGESIZE}, ${COMMONPAGESIZE})"
+ else
+ DATA_SEGMENT_ALIGN="DATA_SEGMENT_ALIGN (${MAXPAGESIZE}, ${COMMONPAGESIZE})"
+ fi
DATA_SEGMENT_END=". = DATA_SEGMENT_END (.);"
DATA_SEGMENT_RELRO_END=". = DATA_SEGMENT_RELRO_END (${SEPARATE_GOTPLT-0}, .);"
fi
--
Alan Modra
Australia Development Lab, IBM