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Fri, 18 Oct 2019 11:40:34 +0000 (UTC) Received: from thuth.remote.csb (dhcp-200-228.str.redhat.com [10.33.200.228]) by smtp.corp.redhat.com (Postfix) with ESMTP id 23CDA5D713; Fri, 18 Oct 2019 11:40:33 +0000 (UTC) Subject: Re: [PATCH] Do not use %m in common code to print error messages To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20191018104438.6158-1-thuth@redhat.com> <20191018105710.GD28271@redhat.com> <4bb84a56-30b1-72e7-7df1-770505744b39@redhat.com> <20191018113703.GE28271@redhat.com> From: Thomas Huth Openpgp: preference=signencrypt Autocrypt: addr=thuth@redhat.com; prefer-encrypt=mutual; keydata= mQINBFH7eUwBEACzyOXKU+5Pcs6wNpKzrlJwzRl3VGZt95VCdb+FgoU9g11m7FWcOafrVRwU yYkTm9+7zBUc0sW5AuPGR/dp3pSLX/yFWsA/UB4nJsHqgDvDU7BImSeiTrnpMOTXb7Arw2a2 4CflIyFqjCpfDM4MuTmzTjXq4Uov1giGE9X6viNo1pxyEpd7PanlKNnf4PqEQp06X4IgUacW tSGj6Gcns1bCuHV8OPWLkf4hkRnu8hdL6i60Yxz4E6TqlrpxsfYwLXgEeswPHOA6Mn4Cso9O 0lewVYfFfsmokfAVMKWzOl1Sr0KGI5T9CpmRfAiSHpthhHWnECcJFwl72NTi6kUcUzG4se81 O6n9d/kTj7pzTmBdfwuOZ0YUSqcqs0W+l1NcASSYZQaDoD3/SLk+nqVeCBB4OnYOGhgmIHNW 0CwMRO/GK+20alxzk//V9GmIM2ACElbfF8+Uug3pqiHkVnKqM7W9/S1NH2qmxB6zMiJUHlTH gnVeZX0dgH27mzstcF786uPcdEqS0KJuxh2kk5IvUSL3Qn3ZgmgdxBMyCPciD/1cb7/Ahazr 3ThHQXSHXkH/aDXdfLsKVuwDzHLVSkdSnZdt5HHh75/NFHxwaTlydgfHmFFwodK8y/TjyiGZ zg2Kje38xnz8zKn9iesFBCcONXS7txENTzX0z80WKBhK+XSFJwARAQABtB5UaG9tYXMgSHV0 aCA8dGh1dGhAcmVkaGF0LmNvbT6JAjgEEwECACIFAlVgX6oCGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAAoJEC7Z13T+cC21EbIP/ii9cvT2HHGbFRl8HqGT6+7Wkb+XLMqJBMAIGiQK QIP3xk1HPTsLfVG0ao4hy/oYkGNOP8+ubLnZen6Yq3zAFiMhQ44lvgigDYJo3Ve59gfe99KX EbtB+X95ODARkq0McR6OAsPNJ7gpEUzfkQUUJTXRDQXfG/FX303Gvk+YU0spm2tsIKPl6AmV 1CegDljzjycyfJbk418MQmMu2T82kjrkEofUO2a24ed3VGC0/Uz//XCR2ZTo+vBoBUQl41BD eFFtoCSrzo3yPFS+w5fkH9NT8ChdpSlbNS32NhYQhJtr9zjWyFRf0Zk+T/1P7ECn6gTEkp5k ofFIA4MFBc/fXbaDRtBmPB0N9pqTFApIUI4vuFPPO0JDrII9dLwZ6lO9EKiwuVlvr1wwzsgq zJTPBU3qHaUO4d/8G+gD7AL/6T4zi8Jo/GmjBsnYaTzbm94lf0CjXjsOX3seMhaE6WAZOQQG tZHAO1kAPWpaxne+wtgMKthyPLNwelLf+xzGvrIKvLX6QuLoWMnWldu22z2ICVnLQChlR9d6 WW8QFEpo/FK7omuS8KvvopFcOOdlbFMM8Y/8vBgVMSsK6fsYUhruny/PahprPbYGiNIhKqz7 UvgyZVl4pBFjTaz/SbimTk210vIlkDyy1WuS8Zsn0htv4+jQPgo9rqFE4mipJjy/iboDuQIN BFH7eUwBEAC2nzfUeeI8dv0C4qrfCPze6NkryUflEut9WwHhfXCLjtvCjnoGqFelH/PE9NF4 4VPSCdvD1SSmFVzu6T9qWdcwMSaC+e7G/z0/AhBfqTeosAF5XvKQlAb9ZPkdDr7YN0a1XDfa +NgA+JZB4ROyBZFFAwNHT+HCnyzy0v9Sh3BgJJwfpXHH2l3LfncvV8rgFv0bvdr70U+On2XH 5bApOyW1WpIG5KPJlDdzcQTyptOJ1dnEHfwnABEfzI3dNf63rlxsGouX/NFRRRNqkdClQR3K gCwciaXfZ7ir7fF0u1N2UuLsWA8Ei1JrNypk+MRxhbvdQC4tyZCZ8mVDk+QOK6pyK2f4rMf/ WmqxNTtAVmNuZIwnJdjRMMSs4W4w6N/bRvpqtykSqx7VXcgqtv6eqoDZrNuhGbekQA0sAnCJ VPArerAZGArm63o39me/bRUQeQVSxEBmg66yshF9HkcUPGVeC4B0TPwz+HFcVhheo6hoJjLq knFOPLRj+0h+ZL+D0GenyqD3CyuyeTT5dGcNU9qT74bdSr20k/CklvI7S9yoQje8BeQAHtdV cvO8XCLrpGuw9SgOS7OP5oI26a0548M4KldAY+kqX6XVphEw3/6U1KTf7WxW5zYLTtadjISB X9xsRWSU+Yqs3C7oN5TIPSoj9tXMoxZkCIHWvnqGwZ7JhwARAQABiQIfBBgBAgAJBQJR+3lM AhsMAAoJEC7Z13T+cC21hPAQAIsBL9MdGpdEpvXs9CYrBkd6tS9mbaSWj6XBDfA1AEdQkBOn ZH1Qt7HJesk+qNSnLv6+jP4VwqK5AFMrKJ6IjE7jqgzGxtcZnvSjeDGPF1h2CKZQPpTw890k fy18AvgFHkVk2Oylyexw3aOBsXg6ukN44vIFqPoc+YSU0+0QIdYJp/XFsgWxnFIMYwDpxSHS 5fdDxUjsk3UBHZx+IhFjs2siVZi5wnHIqM7eK9abr2cK2weInTBwXwqVWjsXZ4tq5+jQrwDK cvxIcwXdUTLGxc4/Z/VRH1PZSvfQxdxMGmNTGaXVNfdFZjm4fz0mz+OUi6AHC4CZpwnsliGV ODqwX8Y1zic9viSTbKS01ZNp175POyWViUk9qisPZB7ypfSIVSEULrL347qY/hm9ahhqmn17 Ng255syASv3ehvX7iwWDfzXbA0/TVaqwa1YIkec+/8miicV0zMP9siRcYQkyTqSzaTFBBmqD oiT+z+/E59qj/EKfyce3sbC9XLjXv3mHMrq1tKX4G7IJGnS989E/fg6crv6NHae9Ckm7+lSs IQu4bBP2GxiRQ+NV3iV/KU3ebMRzqIC//DCOxzQNFNJAKldPe/bKZMCxEqtVoRkuJtNdp/5a yXFZ6TfE1hGKrDBYAm4vrnZ4CXFSBDllL59cFFOJCkn4Xboj/aVxxJxF30bn Organization: Red Hat Message-ID: <431ee20e-6a30-e002-ad8d-de3ff7b38c48@redhat.com> Date: Fri, 18 Oct 2019 13:40:32 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20191018113703.GE28271@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Fri, 18 Oct 2019 11:40:34 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-trivial@nongnu.org, Paolo Bonzini , Kamil Rytarowski , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 18/10/2019 13.37, Daniel P. Berrang=C3=A9 wrote: > On Fri, Oct 18, 2019 at 01:31:34PM +0200, Thomas Huth wrote: >> On 18/10/2019 12.57, Daniel P. Berrang=C3=A9 wrote: >>> On Fri, Oct 18, 2019 at 12:44:38PM +0200, Thomas Huth wrote: >>>> The %m format specifier is an extension from glibc - and when compil= ing >>>> QEMU for NetBSD, the compiler correctly complains, e.g.: >>>> >>>> /home/qemu/qemu-test.ELjfrQ/src/util/main-loop.c: In function 'sigfd= _handler': >>>> /home/qemu/qemu-test.ELjfrQ/src/util/main-loop.c:64:13: warning: %m = is only >>>> allowed in syslog(3) like functions [-Wformat=3D] >>>> printf("read from sigfd returned %zd: %m\n", len); >>>> ^ >>>> Let's use g_strerror() here instead, which is an easy-to-use wrapper >>>> around the thread-safe strerror_r() function. >>>> >>>> While we're at it, also convert the "printf()" in main-loop.c into >>>> the preferred "error_report()". >>>> >>>> Signed-off-by: Thomas Huth >>>> --- >>>> hw/misc/tmp421.c | 8 ++++++-- >>>> util/main-loop.c | 4 +++- >>>> util/systemd.c | 5 +++-- >>>> 3 files changed, 12 insertions(+), 5 deletions(-) >>>> >>>> diff --git a/hw/misc/tmp421.c b/hw/misc/tmp421.c >>>> index 9f044705fa..f23c46a40a 100644 >>>> --- a/hw/misc/tmp421.c >>>> +++ b/hw/misc/tmp421.c >>>> @@ -120,7 +120,9 @@ static void tmp421_get_temperature(Object *obj, = Visitor *v, const char *name, >>>> int tempid; >>>> =20 >>>> if (sscanf(name, "temperature%d", &tempid) !=3D 1) { >>>> - error_setg(errp, "error reading %s: %m", name); >>>> + const char *errmsg =3D g_strerror(errno); >>>> + error_setg(errp, "error reading %s: %s", name, errmsg); >>>> + g_free((gpointer)errmsg); >>> >>> Kaboom crash. This is trying to free a const string that is the calle= r >>> doesn't own. It remains under ownership of g_strerror forever. >> >> Well, if you look at the implementation of g_strerror(), you can see >> that glib returns allocated memory here (with g_strdup() for example). >> So if we don't free, this will leak memory over time. >=20 > This is *not* a leak, it is a cache. >=20 > It is maintaining a static hash table that caches the mapping from > errno -> string. So many calls all with the same errno value will > return the same cached string. So by free'ing the string you're > causing a use-after-free flaw the next time the same errno is > passed to g_strerror. The memory usage here is finite, bounded > by the number of possible errnos that we see. >=20 >> But you're right, the "const" and documentation of the function indica= te >> that the caller should rather not free the memory. What a bummer, soun= ds >> like g_strerror() is pretty much useless unless you immediately exit() >> afterwards (and you care about not leaking memory). >> >> I guess we have to implement our own wrapper, qemu_strerror() instead? >=20 > No, g_strerror is working fine without free'ing the result. Ah, well, thanks for the explanation, not sure how I could have missed that when looking at the source code :-/ Time for weekend, I guess... So g_strerror should be fine, and I'll respin the patch without g_free. Thomas