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Message-ID: <46D67E43.9050704@gmail.com>
Date: Thu, 30 Aug 2007 10:22:27 +0200
From: Franck Bui-Huu <vagabon.xyz@...il.com>
To: Arjan van de Ven <arjan@...radead.org>
CC: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: arch_align_stack() seems useless
Arjan,
Franck Bui-Huu wrote:
> Arjan van de Ven wrote:
>> arch_align_stack aligns, on x86, within a 2 page range (this is for
>> cache coloring).
>
> OK, but for elf case this seems useless since the top of the stack is
> already randomized.
>
> It seems that the randomization stuff (top of the stack + stack
> pointer inside a page) belongs to the elf binary format whereas it
> could have been part of exec.c. Are there any reasons ?
>
>> The other thing you missed is that arch_align_stack()
>> is called in 2 locations, binfmt_elf.c is the primary location for
>> inside-the-page randomization.
>>
>
> Well not really because for mips case, we have:
>
> $ git grep ELF_PLATFORM include/asm-mips
> include/asm-mips/elf.h:#define ELF_PLATFORM (NULL)
>
> So on mips, the stack pointer won't get the inside the page
> randomization. Is that correct ?
>
> If so, I'm wondering why this randomization must depend on that string
> to be defined. I must admit that I'm not sure how it's used. I guess
> it's used by ld.so and it could be set to "mips" for now...
>
Do you think that the diff below would be correct ? I tested it on
mips and it seems to work fine but I can't really say if it's
valid enough to submit it as a patch...
thanks
Franck
---8<---
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 4482a06..024006e 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -151,6 +151,13 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
struct vm_area_struct *vma;
/*
+ * In some cases (e.g. Hyper-Threading), we want to avoid L1
+ * evictions by the processes running on the same package. One
+ * thing we can do is to shuffle the initial stack for them.
+ */
+ p = arch_align_stack(p);
+
+ /*
* If this architecture has a platform capability string, copy it
* to userspace. In some cases (Sparc), this info is impossible
* for userspace to get any other way, in others (i386) it is
@@ -160,14 +167,6 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
if (k_platform) {
size_t len = strlen(k_platform) + 1;
- /*
- * In some cases (e.g. Hyper-Threading), we want to avoid L1
- * evictions by the processes running on the same package. One
- * thing we can do is to shuffle the initial stack for them.
- */
-
- p = arch_align_stack(p);
-
u_platform = (elf_addr_t __user *)STACK_ALLOC(p, len);
if (__copy_to_user(u_platform, k_platform, len))
return -EFAULT;
-
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