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[Qemu-devel] [Bug 1803872] Re: [PATCH v3 5/5] migration: Use strnlen() f
From: |
Eric Blake |
Subject: |
[Qemu-devel] [Bug 1803872] Re: [PATCH v3 5/5] migration: Use strnlen() for fixed-size string |
Date: |
Tue, 18 Dec 2018 19:33:43 -0000 |
On 12/18/18 11:51 AM, Philippe Mathieu-Daudé wrote:
> GCC 8 introduced the -Wstringop-overflow, which detect buffer overflow
> by string-modifying functions declared in <string.h>, such strncpy(),
> used in global_state_store_running().
>
> Since the global_state.runstate does not necessarily contains a
> terminating NUL character, We had to use the QEMU_NONSTRING attribute.
s/We/we/
>
> The GCC manual says about the nonstring attribute:
>
> However, when the array is declared with the attribute the call to
> strlen is diagnosed because when the array doesn’t contain a
> NUL-terminated string the call is undefined. [...]
> In addition, calling strnlen and strndup with such arrays is safe
> provided a suitable bound is specified, and not diagnosed.
>
> GCC indeed found an incorrect use of strlen(), because this array
> is loaded by VMSTATE_BUFFER(runstate, GlobalState) then parsed
> using qapi_enum_parse which does not get the buffer length.
>
> Use strnlen() which returns sizeof(s->runstate) if the array is not
> NUL-terminated.
>
> This fixes:
>
> CC migration/global_state.o
> qemu/migration/global_state.c: In function 'global_state_pre_save':
> qemu/migration/global_state.c:109:15: error: 'strlen' argument 1 declared
> attribute 'nonstring' [-Werror=stringop-overflow=]
> s->size = strlen((char *)s->runstate) + 1;
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> qemu/migration/global_state.c:24:13: note: argument 'runstate' declared
> here
> uint8_t runstate[100] QEMU_NONSTRING;
> ^~~~~~~~
> cc1: all warnings being treated as errors
> make: *** [qemu/rules.mak:69: migration/global_state.o] Error 1
>
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> ---
> migration/global_state.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/migration/global_state.c b/migration/global_state.c
> index 6e19333422..c19030ef62 100644
> --- a/migration/global_state.c
> +++ b/migration/global_state.c
> @@ -106,7 +106,7 @@ static int global_state_pre_save(void *opaque)
> GlobalState *s = opaque;
>
> trace_migrate_global_state_pre_save((char *)s->runstate);
> - s->size = strlen((char *)s->runstate) + 1;
The old code sets s->size to the string length + space for the NUL byte
(by assuming that a NUL byte was present), and accidentally sets it
beyond the s->runstate array if there was no NUL byte (our existing
runstate names are shorter than 100 bytes, so this could only happen on
a malicious stream).
> + s->size = strnlen((char *)s->runstate, sizeof(s->runstate)) + 1;
The new code can still end up setting s->size beyond the array. Is that
intended, or would it be better to use strnlen(s->runstate,
sizeof(s->runstate) - 1) + 1?
Also, as I argued on 4/5, why isn't this squashed in with the patch that
marks the field NONSTRING?
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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https://bugs.launchpad.net/bugs/1803872
Title:
gcc 8.2 reports stringop-truncation when building qemu
Status in QEMU:
New
Bug description:
QEMU 3.0
block/sheepdog.c: In function 'find_vdi_name':
block/sheepdog.c:1239:5: error: 'strncpy' specified bound 256 equals
destination size [-Werror=stringop-truncation]
strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If this is the intended behavior, please suppress the warning. For
example:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstringop-truncation"
strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
#pragma GCC diagnostic pop
This also happens on other sources, for example hw/acpi/core.c, so
another option is to suppress it globally on CFLAGS (-Wno-stringop-
truncation)
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- [Qemu-devel] [Bug 1803872] Re: [PATCH v3 5/5] migration: Use strnlen() for fixed-size string,
Eric Blake <=