Re: [PATCH v2 1/7] util: introduce a parser for kernel cmdline arguments

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On Fri, May 29, 2020 at 12:10:03PM +0200, Paulo de Rezende Pinatti wrote:
> Introduce two utility functions to parse a kernel command
> line string according to the kernel code parsing rules in
> order to enable the caller to perform operations such as
> verifying whether certain argument=value combinations are
> present or retrieving an argument's value.
>
> Signed-off-by: Paulo de Rezende Pinatti <ppinatti@xxxxxxxxxxxxx>
> Signed-off-by: Boris Fiuczynski <fiuczy@xxxxxxxxxxxxx>
> ---
>  src/libvirt_private.syms |   2 +
>  src/util/virutil.c       | 169 +++++++++++++++++++++++++++++++++++++++
>  src/util/virutil.h       |  17 ++++
>  tests/utiltest.c         | 141 ++++++++++++++++++++++++++++++++
>  4 files changed, 329 insertions(+)
>
> diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
> index a6af44fe1c..a206a943c5 100644
> --- a/src/libvirt_private.syms
> +++ b/src/libvirt_private.syms
> @@ -3433,6 +3433,8 @@ virHostGetDRMRenderNode;
>  virHostHasIOMMU;
>  virIndexToDiskName;
>  virIsDevMapperDevice;
> +virKernelCmdlineMatchParam;
> +virKernelCmdlineNextParam;
>  virMemoryLimitIsSet;
>  virMemoryLimitTruncate;
>  virMemoryMaxValue;
> diff --git a/src/util/virutil.c b/src/util/virutil.c
> index fb46501142..749c9d7116 100644
> --- a/src/util/virutil.c
> +++ b/src/util/virutil.c
> @@ -1725,6 +1725,175 @@ virHostGetDRMRenderNode(void)
>      return ret;
>  }
>
> +
> +static const char *virKernelCmdlineSkipDbQuote(const char *cmdline,

minor nitpick: we can call ^this simply ...SkipQuote, we don't need to be so
specific about it being a double quotation mark, do we?

> +                                               bool *is_quoted)
> +{
> +    if (cmdline[0] == '"') {
> +        *is_quoted = !(*is_quoted);
> +        cmdline++;
> +    }
> +    return cmdline;
> +}
> +
> +
> +static size_t virKernelCmdlineSearchForward(const char *cmdline,
> +                                            bool *is_quoted,
> +                                            bool include_equal)

Hmm, what if instead we tried to find and return the index of the '=' character
but iterated all the way until the next applicable (i.e. taking quotation into
account) space and saved that end of arg/arg=val parameter into **res. The
caller of this function would then continue directly from *res with the next
arg/arg=val parameter.
We could then call this something like virKernelCmdlineFindEqual and return -1
if there is no '=' sign, indicating that it's a standalone parameter with no
value.

> +{
> +    size_t index;
> +
> +    for (index = 0; cmdline[index]; index++) {
> +        if ((!(*is_quoted) && g_ascii_isspace(cmdline[index])) ||
> +            (include_equal && cmdline[index] == '='))
> +            break;
> +        virKernelCmdlineSkipDbQuote(cmdline + index, is_quoted);
> +    }
> +    return index;
> +}
> +
> +
> +static size_t virKernelCmdlineNextSpace(const char *cmdline,
> +                                        bool *is_quoted)
> +{
> +    return virKernelCmdlineSearchForward(cmdline, is_quoted, false);
> +}
> +
> +
> +static size_t virKernelCmdlineNextSpaceOrEqual(const char *cmdline,
> +                                               bool *is_quoted)
> +{
> +    return virKernelCmdlineSearchForward(cmdline, is_quoted, true);
> +}

If we implement what I suggested above for virKernelCmdlineSearchForward, we
won't need 2 wrappers for the same thing differentiating only in whether we do
or do not need to search for the '=' sign as well.

> +
> +
> +static char* virKernelArgNormalize(const char *arg)
> +{
> +    return virStringReplace(arg, "_", "-");
> +}
> +
> +
> +static char* virKernelCmdlineArgNormalize(const char *cmdline, size_t offset)
> +{
> +    g_autofree char *param = g_strndup((cmdline), offset);
> +
> +    return virKernelArgNormalize(param);

See below for comments why we don't need this wrapper.

> +}
> +
> +
> +/*
> + * Parse the kernel cmdline and store the next parameter in @param
> + * and the value of @param in @val which can be NULL if @param has
> + * no value. In addition returns the address right after @param=@value
> + * for possible further processing.
> + *
> + * @cmdline: kernel command line string to be checked for next parameter
> + * @param: pointer to hold retrieved parameter, will be NULL if none found
> + * @val: pointer to hold retrieved value of @param
> + *
> + * Returns a pointer to address right after @param=@val in the
> + * kernel command line, will point to the string's end (NULL)
> + * in case no next parameter is found
> + */
> +const char *virKernelCmdlineNextParam(const char *cmdline,
> +                                      char **param,
> +                                      char **val)
> +{
> +    size_t offset;
> +    bool is_quoted = false;
> +    *param = NULL;
> +    *val = NULL;
> +
> +    virSkipSpaces(&cmdline);
> +    cmdline = virKernelCmdlineSkipDbQuote(cmdline, &is_quoted);
> +    offset = virKernelCmdlineNextSpaceOrEqual(cmdline, &is_quoted);

if you get the index of the equal sign, but iterate all the way until the end
of the arg/arg=val parameter, you can use index and do:

*param = g_strndup(cmdline, equal_index);

> +    if (offset == 0)

If we used something like char **res (taking it as a parameter to this function)
to represent the rest of the unparsed cmdline, then it should be NULL in this
case, so the check could remain with a slight adjustment (I haven't implemented
it, but I do hope it should work :))

> +        return cmdline;
> +
> +    *param = virKernelCmdlineArgNormalize(cmdline, offset);

^This normalization should be done in virKernelCmdlineMatchParam instead. That
way, you won't need a wrapper over virKernelArgNormalize.

> +    cmdline = cmdline + offset;
> +    /* param has no value */
> +    if (*cmdline != '=')
> +        return cmdline;

^This check could then be dropped along with moving the cursor in the cmdline
string.

> +
> +    cmdline = virKernelCmdlineSkipDbQuote(++cmdline, &is_quoted);
> +    offset = virKernelCmdlineNextSpace(cmdline, &is_quoted);

If we do the above, we should be able to ditch ^this second loop searching for
the next space.

> +    if (cmdline[offset-1] == '"')
> +        *val = g_strndup(cmdline, offset-1);
> +    else
> +        *val = g_strndup(cmdline, offset);

^Here you'd have to do arithmetics using the *res variable instead.

> +
> +    return cmdline + offset;

We'd simply return *res;

> +}
> +
> +
> +#define VIR_CMDLINE_STR_CMP(kernel_val, caller_val, flags) \
> +    (((flags & VIR_KERNEL_CMDLINE_FLAGS_CMP_EQ) && \
> +      STREQ(kernel_val, caller_val)) || ((flags & VIR_KERNEL_CMDLINE_FLAGS_CMP_PREFIX) && \
> +                                         STRPREFIX(kernel_val, caller_val)))
> +
> +
> +/*
> + * Try to match the provided kernel cmdline string with the provided @arg
> + * and the list @values of possible values according to the matching strategy
> + * defined in @flags. Possible options include:
> + * - VIR_KERNEL_CMDLINE_FLAGS_CMP_PREFIX: do a substring comparison of values

I know you used it in the following patches to match against the accepted
values, but do we really need to match with a prefix, I'd be fine with simple
stirng equality matching in all cases and not mix the matching strategy for
both values and kernel arguments.

> + *   (uses size of value provided as input)
> + * - VIR_KERNEL_CMDLINE_FLAGS_CMP_EQ: do a strict string comparison of values

^This should be default used in all cases IMO unless the prefix matching
provides us with a considerable performance gain.

> + * - VIR_KERNEL_CMDLINE_FLAGS_SEARCH_STICKY: first positive match satifies search

is "sticky searching" some terminus technicus? If not, we probably should name
this FLAGS_MATCH_FIRST and FLAGS_MATCH_LAST respectively.

> + *   (in case of multiple argument occurrences)
> + * - VIR_KERNEL_CMDLINE_FLAGS_SEARCH_LAST: use the result of last argument occurence
> + *   (in case of multiple argument occurrences)
> + *
> + * @cmdline: kernel command line string to be checked for @arg
> + * @arg: kernel command line argument
> + * @values: array of possible values to match @arg
> + * @len_values: size of array, it can be 0 meaning a match will be positive if the
> + * argument has no value.
> + * @flags: flag mask defining the strategy for matching and comparing
> + *
> + * Returns true if a match is found, false otherwise
> + */
> +bool virKernelCmdlineMatchParam(const char *cmdline,
> +                                const char *arg,
> +                                const char **values,
> +                                size_t len_values,
> +                                virKernelCmdlineFlags flags)
> +{
> +    bool match = false;
> +    size_t i;
> +    const char *next = cmdline;
> +    g_autofree char *norm_arg = virKernelArgNormalize(arg);
> +    g_autofree char *kparam = NULL;
> +    g_autofree char *kval = NULL;

^These last two variables can be moved into the while loop, you won't need the
explicit VIR_FREEs then.

> +
> +    while (next[0] != '\0') {
> +        VIR_FREE(kparam);
> +        VIR_FREE(kval);
> +        next = virKernelCmdlineNextParam(next, &kparam, &kval);

Insert a blank line in between all these "ifs" for better readability (I know
the coding guideline we have doesn't mention it).

> +        if (!kparam)
> +            break;

You'd do the normalization of the parsed arg value here.

> +        if (STRNEQ(kparam, norm_arg))
> +            continue;
> +        if (!kval) {
> +            match = (len_values == 0) ? true : false;
> +        } else {
> +            match = false;
> +            for (i = 0; i < len_values; i++) {
> +                if (VIR_CMDLINE_STR_CMP(kval, values[i], flags)) {
> +                    match = true;
> +                    break;
> +                }
> +            }
> +        }
> +        if (match && (flags & VIR_KERNEL_CMDLINE_FLAGS_SEARCH_STICKY))
> +            break;
> +    }
> +
> +    return match;
> +}
> +
> +
>  /*
>   * Get a password from the console input stream.
>   * The caller must free the returned password.
> diff --git a/src/util/virutil.h b/src/util/virutil.h
> index 49b4bf440f..7499b78153 100644
> --- a/src/util/virutil.h
> +++ b/src/util/virutil.h
> @@ -147,6 +147,23 @@ bool virHostHasIOMMU(void);
>
>  char *virHostGetDRMRenderNode(void) G_GNUC_NO_INLINE;
>
> +typedef enum {
> +    VIR_KERNEL_CMDLINE_FLAGS_CMP_PREFIX = 1,
> +    VIR_KERNEL_CMDLINE_FLAGS_CMP_EQ = 2,
> +    VIR_KERNEL_CMDLINE_FLAGS_SEARCH_STICKY = 4,
> +    VIR_KERNEL_CMDLINE_FLAGS_SEARCH_LAST = 8,
> +} virKernelCmdlineFlags;
> +
> +const char *virKernelCmdlineNextParam(const char *cmdline,
> +                                      char **param,
> +                                      char **val);
> +
> +bool virKernelCmdlineMatchParam(const char *cmdline,
> +                                const char *arg,
> +                                const char **values,
> +                                size_t len_values,
> +                                virKernelCmdlineFlags flags);
> +
>  /**
>   * VIR_ASSIGN_IS_OVERFLOW:
>   * @rvalue: value that is checked (evaluated twice)
> diff --git a/tests/utiltest.c b/tests/utiltest.c
> index 5ae04585cb..01fb8c89f5 100644
> --- a/tests/utiltest.c
> +++ b/tests/utiltest.c
> @@ -254,6 +254,145 @@ testOverflowCheckMacro(const void *data G_GNUC_UNUSED)
>  }
>
>
> +struct testKernelCmdlineNextParamData
> +{
> +    const char *cmdline;
> +    const char *param;
> +    const char *val;
> +    const char *next;
> +};
> +
> +static struct testKernelCmdlineNextParamData kEntries[] = {
> +    { "arg1 arg2 arg3=val1",                            "arg1",           NULL,                  " arg2 arg3=val1" },
> +    { "arg1=val1 arg2 arg3=val3 arg4",                  "arg1",           "val1",                " arg2 arg3=val3 arg4" },
> +    { "arg3=val3 ",                                     "arg3",           "val3",                " " },
> +    { "arg3=val3",                                      "arg3",           "val3",                "" },
> +    { "arg-3=val3 arg4",                                "arg-3",          "val3",                " arg4" },
> +    { "arg_3=val3 arg4",                                "arg-3",          "val3",                " arg4" },
> +    { " arg_3=val3 arg4",                               "arg-3",          "val3",                " arg4" },
> +    { "  arg-3=val3 arg4",                              "arg-3",          "val3",                " arg4" },
> +    { "arg2=\"value with spaces\" arg3=val3",           "arg2",           "value with spaces",   " arg3=val3" },
> +    { " arg2=\"value with spaces\"   arg3=val3",        "arg2",           "value with spaces",   "   arg3=val3" },
> +    { "  \"arg2=value with spaces\" arg3=val3",         "arg2",           "value with spaces",   " arg3=val3" },
> +    { "arg2=\"val\"ue arg3",                            "arg2",           "val\"ue",             " arg3" },
> +    { " arg3\" escaped=val2\"",                         "arg3\" escaped", "val2",                "" },

^Is this even valid for the kernel itself? Looking at [1], they clearly don't
allow escaped \" in the arg/value.

[1] https://github.com/torvalds/linux/blob/db54615e21419c3cb4d699a0b0aa16cc44d0e9da/lib/cmdline.c

> +    { " arg2longer=someval arg2=val2 arg3 ",            "arg2longer",     "someval",             " arg2=val2 arg3 " },
> +    { "=val1 arg2=val2",                                NULL,             NULL,                  "=val1 arg2=val2" },
> +    { " ",                                              NULL,             NULL,                  "" },
> +    { "",                                               NULL,             NULL,                  "" },
> +};
> +
> +static int
> +testKernelCmdlineNextParam(const void *data G_GNUC_UNUSED)
> +{
> +    char *param = NULL;
> +    char *val = NULL;
> +    const char *next;
> +    size_t i;
> +
> +    for (i = 0; i < G_N_ELEMENTS(kEntries); ++i) {
> +        VIR_FREE(param);
> +        VIR_FREE(val);
> +
> +        next = virKernelCmdlineNextParam(kEntries[i].cmdline, &param, &val);
> +
> +        if (STRNEQ_NULLABLE(param, kEntries[i].param) ||
> +            STRNEQ_NULLABLE(val, kEntries[i].val) ||
> +            STRNEQ(next, kEntries[i].next)) {
> +            VIR_TEST_DEBUG("\nKernel cmdline [%s]", kEntries[i].cmdline);
> +            VIR_TEST_DEBUG("Expect param [%s]", kEntries[i].param);
> +            VIR_TEST_DEBUG("Actual param [%s]", param);
> +            VIR_TEST_DEBUG("Expect value [%s]", kEntries[i].val);
> +            VIR_TEST_DEBUG("Actual value [%s]", val);
> +            VIR_TEST_DEBUG("Expect next [%s]", kEntries[i].next);
> +            VIR_TEST_DEBUG("Actual next [%s]", next);
> +
> +            VIR_FREE(param);
> +            VIR_FREE(val);
> +
> +            return -1;
> +        }
> +    }
> +
> +    VIR_FREE(param);
> +    VIR_FREE(val);
> +
> +    return 0;
> +}

I appreciate the thorough unit testing :)

Erik




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