Re: [Bug 1895080] [NEW] pgb_reserved_va: Assertion `addr == test' failed

[Alex Bennée]

Hansni Bu <1895080@bugs.launchpad.net> writes:

> No, it's not set by CentOS-7.5.
> Does it mean that we just cannot run the ELF in such a case? I've tried
> many times, the assert always fails. Maybe, we can blame CentOS-7.5.

The trouble is without MAP_FIXED_NOREPLACE we are at the mercy of the
host kernel to allow the address request to be honoured. A plain
MAP_FIXED won't do as it can clober existing mappings. In theory a
suitable hole has been identified but sometimes the kernel makes a
decision to offset the suggested mapping for it's own reasons. 

> BTW: with the option "-p 65536", the case runs successfully.

Hmm interesting. I wonder if we are seeing a fail due to mmap_min_addr?
What does:

  /proc/sys/vm/mmap_min_addr

give you on the system?

You can manually set the reserved_va and the base address using -R and
-B although that is more of a developer work around. I think moving the
assert to the condition above would be an improvement just because it
tells us what the requested base address was and what the kernel decided
to give us.

>
> On Fri, Sep 11, 2020 at 5:50 PM Alex Bennée <1895080@bugs.launchpad.net>
> wrote:
>
>> Alex Bennée <alex.bennee@linaro.org> writes:
>>
>> > Laurent Vivier <laurent@vivier.eu> writes:
>> >
>> <snip>
>> >>> Then trying qemu-riscv32 with a simple ELF, I get:
>> >>> linux-user/elfload.c:2341: pgb_reserved_va: Assertion `addr == test'
>> failed.
>> >>>
>> >>> strace shows that:
>> >>> mmap(0x1000, 4294963200, PROT_NONE,
>> MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x10000
>> >>> write(2, "qemu-riscv32: ../qemu.git/linux-"..., 103qemu-riscv32:
>> ../qemu.git/linux-user/elfload.c:2341: pgb_reserved_va: Assertion `addr ==
>> test' failed.
>> >>> ) = 103
>> >>>
>> >>> The source code is in the function pgb_reserved_va (linux-
>> >>> user/elfload.c). I think mmap cannot guarantee that the returned
>> pointer
>> >>> (test) equals to the parameter of addr. So is this a bug to assert
>> (addr
>> >>> == test)?
>> >>
>> > I'm assuming CentOS 7.5 actually has a definition for
>> > MAP_FIXED_NOREPLACE which should ensure we get what we asked for -
>> > otherwise we are in the position of hoping the kernel honours what we
>> > asked for.
>>
>> Doh re-reading I see it's not set in the strace output. Maybe we should
>> promote the assert case to the failure leg so we have:
>>
>>     if (addr == MAP_FAILED || addr != test) {
>>         error_report(...)
>>     }
>>
>> so we at least fail with a user friendly error rather than an abort?
>>
>> --
>> Alex Bennée
>>
>> --
>> You received this bug notification because you are subscribed to the bug
>> report.
>> https://bugs.launchpad.net/bugs/1895080
>>
>> Title:
>>   pgb_reserved_va: Assertion `addr == test' failed
>>
>> Status in QEMU:
>>   New
>>
>> Bug description:
>>   This problem occurs on CentOS-7.5 (64-bit) with qemu-5.1.0, qemu head
>>   (commit 9435a8b3dd35f1f926f1b9127e8a906217a5518a) for riscv32-linux-
>>   user.
>>
>>   Firstly, compile fails:
>>   Compiling C object libqemu-riscv32-linux-user.fa.p/linux-user_strace.c.o
>>   ../qemu.git/linux-user/strace.c:1210:18: error: ‘FALLOC_FL_KEEP_SIZE’
>> undeclared here (not in a function)
>>        FLAG_GENERIC(FALLOC_FL_KEEP_SIZE),
>>
>>   I have to add below include to linux-user/strace.c
>>   diff --git a/linux-user/strace.c b/linux-user/strace.c
>>   index 11fea14fba..22e51d4a8a 100644
>>   --- a/linux-user/strace.c
>>   +++ b/linux-user/strace.c
>>   @@ -7,6 +7,7 @@
>>    #include <sys/mount.h>
>>    #include <arpa/inet.h>
>>    #include <netinet/tcp.h>
>>   +#include <linux/falloc.h>
>>    #include <linux/if_packet.h>
>>    #include <linux/netlink.h>
>>    #include <sched.h>
>>
>>   Then trying qemu-riscv32 with a simple ELF, I get:
>>   linux-user/elfload.c:2341: pgb_reserved_va: Assertion `addr == test'
>> failed.
>>
>>   strace shows that:
>>   mmap(0x1000, 4294963200, PROT_NONE,
>> MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x10000
>>   write(2, "qemu-riscv32: ../qemu.git/linux-"..., 103qemu-riscv32:
>> ../qemu.git/linux-user/elfload.c:2341: pgb_reserved_va: Assertion `addr ==
>> test' failed.
>>   ) = 103
>>
>>   The source code is in the function pgb_reserved_va (linux-
>>   user/elfload.c). I think mmap cannot guarantee that the returned
>>   pointer (test) equals to the parameter of addr. So is this a bug to
>>   assert (addr == test)?
>>
>>   Attached configure script and test ELF file.
>>
>>   Thanks.
>>
>> To manage notifications about this bug go to:
>> https://bugs.launchpad.net/qemu/+bug/1895080/+subscriptions
>>


-- 
Alex Bennée