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Re: [PATCH 1/2] MIPS: Preliminary vdso.

To: David VomLehn <dvomlehn@cisco.com>
Subject: Re: [PATCH 1/2] MIPS: Preliminary vdso.
From: David Daney <ddaney@caviumnetworks.com>
Date: Wed, 22 Apr 2009 11:05:22 -0700
Cc: linux-mips@linux-mips.org, ralf@linux-mips.org
In-reply-to: <20090422175044.GA28623@cuplxvomd02.corp.sa.net>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <49EE3B0F.3040506@caviumnetworks.com> <1240349605-1921-1-git-send-email-ddaney@caviumnetworks.com> <20090422175044.GA28623@cuplxvomd02.corp.sa.net>
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David VomLehn wrote:
On Tue, Apr 21, 2009 at 02:33:24PM -0700, David Daney wrote:
This is a preliminary patch to add a vdso to all user processes.
...
diff --git a/arch/mips/kernel/vdso.c b/arch/mips/kernel/vdso.c
new file mode 100644
index 0000000..a1f38a6
...
+static void __init install_trampoline(u32 *tramp, unsigned int sigreturn)
+{
+       tramp[0] = 0x24020000 + sigreturn;      /* li v0, sigreturn */
+       tramp[1] = 0x0000000c;                  /* syscall */
+}

We can do away with magic constants by building on asm/inst.h:

static void __init install_trampoline(union mips_instruction *tramp,
        short sigreturn)
{
        static struct i_format li_sigreturn = { /* li, v0, sigreturn */
                .opcode = addiu_op,
                .rs = 0,                        /* zero */
                .rt = 2,                        /* v0 */
        };
        static struct r_format syscall = {              /* syscall */
                .opcode = spec_op,
                .func = syscall_op
        };
        tramp[0].i_format = li_sigreturn;
        tramp[0].i_format.simmediate = sigreturn;
        tramp[1].r_format = syscall;
}

Admittedly, this isn't really a proper use of the r_format structure. The
right way is to add another structure to asm/inst.h and use that:

Perhaps using the uasm would be even better.

[...]

+int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
+{
+       int ret;
+       unsigned long addr;
+       struct mm_struct *mm = current->mm;
+
+       down_write(&mm->mmap_sem);
+
+       addr = vdso_addr(mm->start_stack);
+
+       addr = get_unmapped_area(NULL, addr, PAGE_SIZE, 0, 0);
+       if (IS_ERR_VALUE(addr)) {
+               ret = addr;
+               goto up_fail;
+       }
+
+       ret = install_special_mapping(mm, addr, PAGE_SIZE,
+                                     VM_READ|VM_EXEC|
+                                     VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC|
+                                     VM_ALWAYSDUMP,
+                                     &vdso_page);
+
+       if (ret)
+               goto up_fail;
+
+       mm->context.vdso = (void *)addr;
+
+up_fail:

It seems that this is an unexpected condition that probably indicates
a failure of the expected state of the process at this point. Perhaps
a pr_err() or pr_warning() would be appropriate?

+       up_write(&mm->mmap_sem);
+       return ret;
+}

Really it should always succeed. Something is seriously wrong if you cannot map that page and we should probably panic().

David Daney



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