[Qemu-devel] [Bug 597575] [NEW] Hangs on HTTP errors when using the curl block driver

[Apollon Oikonomopoulos]

Public bug reported:

Hi,

It seems that qemu-kvm does not handle HTTP errors gracefully when using
the curl block driver and a synchronous request is made (i.e. one using
bdrv_read_em() for example). In these cases, if an HTTP error (such as
404 or 416) is returned, the aio thread exits but the main thread never
finishes waiting for I/O completion, thus freezing KVM.

Versions affected:
At least 0.11.1 and 0.12.4 were tested and found to be affected.

How to reproduce:
Simply specify a non-existing path for an HTTP URL as a CDROM drive.
kvm -drive file=test.img,format=qcow2,if=ide,index=0,boot=on -drive 
file=http://127.0.0.1/static/test1.iso,media=cdrom,index=2,if=ide -boot c

qemu-kvm will hang on boot using 100% cpu as it will try to open the
block device. At that point, the backtrace is (qemu-kvm-0.12.4):

#0  0x000000000047aaaf in qemu_aio_wait () at aio.c:163
#1  0x000000000047a055 in bdrv_read_em (bs=0x1592320, sector_num=0, 
buf=0x7fffcf7e9ae0 "¨\237~Ïÿ\177", nb_sectors=4)
    at block.c:1939
#2  0x0000000000479c0e in bdrv_pread (bs=0x1592320, offset=<value optimized 
out>, buf=0x7fffcf7e9ae0, count1=2048)
    at block.c:716
#3  0x000000000047a862 in bdrv_open2 (bs=0x1591a30, filename=0x1559f00 
"http://127.0.0.1/static/test1.iso";, 
    flags=0, drv=0x84eca0) at block.c:316
#4  0x000000000040dcb4 in drive_init (opts=0x1559e60, opaque=<value optimized 
out>, fatal_error=0x7fffcf7ea494)
    at 
/build/buildd-qemu-kvm_0.12.4+dfsg-1~bpo50+1-amd64-KOah5G/qemu-kvm-0.12.4+dfsg/vl.c:2471
#5  0x000000000040e086 in drive_init_func (opts=0x155db00, opaque=0x0)
    at 
/build/buildd-qemu-kvm_0.12.4+dfsg-1~bpo50+1-amd64-KOah5G/qemu-kvm-0.12.4+dfsg/vl.c:2488
#6  0x0000000000475421 in qemu_opts_foreach (list=<value optimized out>, 
func=0x40e070 <drive_init_func>, 
    opaque=0x8495e0, abort_on_failure=12) at qemu-option.c:817
#7  0x000000000040e9af in main (argc=7, argv=0x7fffcf7ea838, envp=<value 
optimized out>)
    at 
/build/buildd-qemu-kvm_0.12.4+dfsg-1~bpo50+1-amd64-KOah5G/qemu-kvm-0.12.4+dfsg/vl.c:6011

Thanks

** Affects: qemu
     Importance: Undecided
         Status: New

-- 
Hangs on HTTP errors when using the curl block driver
https://bugs.launchpad.net/bugs/597575
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devel-ml, which is subscribed to QEMU.

Status in QEMU: New

Bug description:
Hi,

It seems that qemu-kvm does not handle HTTP errors gracefully when using the 
curl block driver and a synchronous request is made (i.e. one using 
bdrv_read_em() for example). In these cases, if an HTTP error (such as 404 or 
416) is returned, the aio thread exits but the main thread never finishes 
waiting for I/O completion, thus freezing KVM.

Versions affected:
At least 0.11.1 and 0.12.4 were tested and found to be affected.

How to reproduce:
Simply specify a non-existing path for an HTTP URL as a CDROM drive.
kvm -drive file=test.img,format=qcow2,if=ide,index=0,boot=on -drive 
file=http://127.0.0.1/static/test1.iso,media=cdrom,index=2,if=ide -boot c

qemu-kvm will hang on boot using 100% cpu as it will try to open the block 
device. At that point, the backtrace is (qemu-kvm-0.12.4):

#0  0x000000000047aaaf in qemu_aio_wait () at aio.c:163
#1  0x000000000047a055 in bdrv_read_em (bs=0x1592320, sector_num=0, 
buf=0x7fffcf7e9ae0 "¨\237~Ïÿ\177", nb_sectors=4)
    at block.c:1939
#2  0x0000000000479c0e in bdrv_pread (bs=0x1592320, offset=<value optimized 
out>, buf=0x7fffcf7e9ae0, count1=2048)
    at block.c:716
#3  0x000000000047a862 in bdrv_open2 (bs=0x1591a30, filename=0x1559f00 
"http://127.0.0.1/static/test1.iso";, 
    flags=0, drv=0x84eca0) at block.c:316
#4  0x000000000040dcb4 in drive_init (opts=0x1559e60, opaque=<value optimized 
out>, fatal_error=0x7fffcf7ea494)
    at 
/build/buildd-qemu-kvm_0.12.4+dfsg-1~bpo50+1-amd64-KOah5G/qemu-kvm-0.12.4+dfsg/vl.c:2471
#5  0x000000000040e086 in drive_init_func (opts=0x155db00, opaque=0x0)
    at 
/build/buildd-qemu-kvm_0.12.4+dfsg-1~bpo50+1-amd64-KOah5G/qemu-kvm-0.12.4+dfsg/vl.c:2488
#6  0x0000000000475421 in qemu_opts_foreach (list=<value optimized out>, 
func=0x40e070 <drive_init_func>, 
    opaque=0x8495e0, abort_on_failure=12) at qemu-option.c:817
#7  0x000000000040e9af in main (argc=7, argv=0x7fffcf7ea838, envp=<value 
optimized out>)
    at 
/build/buildd-qemu-kvm_0.12.4+dfsg-1~bpo50+1-amd64-KOah5G/qemu-kvm-0.12.4+dfsg/vl.c:6011

Thanks