[PATCH 6/9] sparc32: colour-align private file mappings

[Magnus Lindholm <linmag7@gmail.com>]

arch_get_unmapped_area() aligns MAP_SHARED so a file page is mapped at the
colour its page offset implies, but gives MAP_PRIVATE an align_mask of
zero. A private file mapping therefore lands at an arbitrary colour, and
on a virtually indexed cache it aliases any other mapping of the same
page.

Measured on an SS-20 with a 512 KB VAC: every page of a private file
mapping sat at the wrong colour, 64 of 64, at three different offsets.

Align them as sparc64 does, whenever there is a file rather than only when
the mapping is shared. MAP_FIXED is left alone: the ELF loader places
segments with MAP_PRIVATE|MAP_FIXED at addresses that need not preserve
colour, so rejecting those makes execve() fail.

Reference: MBus Interface Specification, SPARC International, 1997, signal
VA[19:12] (MAD[53:46]).

This changes alignment on HyperSPARC, where SHMLBA follows the VAC size.
Each private file mapping can consume up to SHMLBA of extra virtual space.

Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/sparc/kernel/sys_sparc_32.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/sparc/kernel/sys_sparc_32.c b/arch/sparc/kernel/sys_sparc_32.c
index fb31bc0c5b48..e1530cee15b2 100644
--- a/arch/sparc/kernel/sys_sparc_32.c
+++ b/arch/sparc/kernel/sys_sparc_32.c
@@ -40,6 +40,15 @@ SYSCALL_DEFINE0(getpagesize)
 	return PAGE_SIZE; /* Possibly older binaries want 8192 on sun4's? */
 }
 
+/*
+ * A file page reached at two colours is two cache lines, so a kernel-chosen
+ * mapping of one sits at the colour its page offset implies - private too.
+ */
+static bool sparc_needs_colour_align(struct file *filp, unsigned long flags)
+{
+	return filp || (flags & MAP_SHARED);
+}
+
 unsigned long arch_get_unmapped_area(struct file *filp, unsigned long addr, unsigned long len, unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
 {
 	struct vm_unmapped_area_info info = {};
@@ -50,7 +59,11 @@ unsigned long arch_get_unmapped_area(struct file *filp, unsigned long addr, unsi
 
 	if (flags & MAP_FIXED) {
 		/* We do not accept a shared mapping if it would violate
-		 * cache aliasing constraints.
+		 * cache aliasing constraints.  Private file mappings are not
+		 * rejected here: the ELF loader places segments with
+		 * MAP_FIXED at offsets that need not preserve colour, and
+		 * refusing them makes execve() fail.  sparc64 draws the line
+		 * in the same place.
 		 */
 		if (!file_hugepage && (flags & MAP_SHARED) &&
 		    ((addr - (pgoff << PAGE_SHIFT)) & (SHMLBA - 1)))
@@ -68,7 +81,7 @@ unsigned long arch_get_unmapped_area(struct file *filp, unsigned long addr, unsi
 	info.low_limit = addr;
 	info.high_limit = TASK_SIZE;
 	if (!file_hugepage) {
-		info.align_mask = (flags & MAP_SHARED) ?
+		info.align_mask = sparc_needs_colour_align(filp, flags) ?
 			(PAGE_MASK & (SHMLBA - 1)) : 0;
 		info.align_offset = pgoff << PAGE_SHIFT;
 	} else {
-- 
2.43.0