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Blame SOURCES/0477-kern-misc-Add-function-to-check-printf-format-agains.patch

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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Thomas Frauendorfer | Miray Software <tf@miray.de>
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Date: Thu, 4 Feb 2021 19:02:33 +0100
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Subject: [PATCH] kern/misc: Add function to check printf() format against
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 expected format
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The grub_printf_fmt_check() function parses the arguments of an untrusted
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printf() format and an expected printf() format and then compares the
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arguments counts and arguments types. The arguments count in the untrusted
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format string must be less or equal to the arguments count in the expected
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format string and both arguments types must match.
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To do this the parse_printf_arg_fmt() helper function is extended in the
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following way:
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  1. Add a return value to report errors to the grub_printf_fmt_check().
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  2. Add the fmt_check argument to enable stricter format verification:
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     - the function expects that arguments definitions are always
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       terminated by a supported conversion specifier.
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     - positional parameters, "$", are not allowed, as they cannot be
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       validated correctly with the current implementation. For example
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       "%s%1$d" would assign the first args entry twice while leaving the
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       second one unchanged.
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     - Return an error if preallocated space in args is too small and
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       allocation fails for the needed size. The grub_printf_fmt_check()
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       should verify all arguments. So, if validation is not possible for
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       any reason it should return an error.
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     This also adds a case entry to handle "%%", which is the escape
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     sequence to print "%" character.
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  3. Add the max_args argument to check for the maximum allowed arguments
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     count in a printf() string. This should be set to the arguments count
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     of the expected format. Then the parse_printf_arg_fmt() function will
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     return an error if the arguments count is exceeded.
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The two additional arguments allow us to use parse_printf_arg_fmt() in
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printf() and grub_printf_fmt_check() calls.
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When parse_printf_arg_fmt() is used by grub_printf_fmt_check() the
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function parse user provided untrusted format string too. So, in
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that case it is better to be too strict than too lenient.
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Signed-off-by: Thomas Frauendorfer | Miray Software <tf@miray.de>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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---
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 grub-core/kern/misc.c | 82 ++++++++++++++++++++++++++++++++++++++++++++++++---
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 include/grub/misc.h   | 16 ++++++++++
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 2 files changed, 94 insertions(+), 4 deletions(-)
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diff --git a/grub-core/kern/misc.c b/grub-core/kern/misc.c
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index 07456faa2a7..859d71659a2 100644
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--- a/grub-core/kern/misc.c
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+++ b/grub-core/kern/misc.c
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@@ -711,8 +711,26 @@ grub_lltoa (char *str, int c, unsigned long long n)
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   return p;
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 }
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-static void
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-parse_printf_arg_fmt (const char *fmt0, struct printf_args *args)
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+/*
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+ * Parse printf() fmt0 string into args arguments.
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+ *
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+ * The parsed arguments are either used by a printf() function to format the fmt0
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+ * string or they are used to compare a format string from an untrusted source
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+ * against a format string with expected arguments.
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+ *
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+ * When the fmt_check is set to !0, e.g. 1, then this function is executed in
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+ * printf() format check mode. This enforces stricter rules for parsing the
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+ * fmt0 to limit exposure to possible errors in printf() handling. It also
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+ * disables positional parameters, "$", because some formats, e.g "%s%1$d",
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+ * cannot be validated with the current implementation.
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+ *
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+ * The max_args allows to set a maximum number of accepted arguments. If the fmt0
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+ * string defines more arguments than the max_args then the parse_printf_arg_fmt()
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+ * function returns an error. This is currently used for format check only.
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+ */
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+static grub_err_t
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+parse_printf_arg_fmt (const char *fmt0, struct printf_args *args,
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+		      int fmt_check, grub_size_t max_args)
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 {
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   const char *fmt;
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   char c;
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@@ -739,7 +757,12 @@ parse_printf_arg_fmt (const char *fmt0, struct printf_args *args)
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 	fmt++;
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       if (*fmt == '$')
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-	fmt++;
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+	{
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+	  if (fmt_check)
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+	    return grub_error (GRUB_ERR_BAD_ARGUMENT,
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+			       "positional arguments are not supported");
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+	  fmt++;
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+	}
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       if (*fmt =='-')
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 	fmt++;
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@@ -771,9 +794,19 @@ parse_printf_arg_fmt (const char *fmt0, struct printf_args *args)
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 	case 's':
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 	  args->count++;
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 	  break;
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+	case '%':
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+	  /* "%%" is the escape sequence to output "%". */
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+	  break;
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+	default:
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+	  if (fmt_check)
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+	    return grub_error (GRUB_ERR_BAD_ARGUMENT, "unexpected format");
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+	  break;
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 	}
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     }
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+  if (fmt_check && args->count > max_args)
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+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "too many arguments");
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+
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   if (args->count <= ARRAY_SIZE (args->prealloc))
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     args->ptr = args->prealloc;
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   else
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@@ -781,6 +814,9 @@ parse_printf_arg_fmt (const char *fmt0, struct printf_args *args)
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       args->ptr = grub_calloc (args->count, sizeof (args->ptr[0]));
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       if (!args->ptr)
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 	{
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+	  if (fmt_check)
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+	    return grub_errno;
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+
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 	  grub_errno = GRUB_ERR_NONE;
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 	  args->ptr = args->prealloc;
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 	  args->count = ARRAY_SIZE (args->prealloc);
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@@ -872,6 +908,8 @@ parse_printf_arg_fmt (const char *fmt0, struct printf_args *args)
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 	  break;
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 	}
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     }
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+
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+  return GRUB_ERR_NONE;
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 }
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 static void
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@@ -879,7 +917,7 @@ parse_printf_args (const char *fmt0, struct printf_args *args, va_list args_in)
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 {
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   grub_size_t n;
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-  parse_printf_arg_fmt (fmt0, args);
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+  parse_printf_arg_fmt (fmt0, args, 0, 0);
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   for (n = 0; n < args->count; n++)
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     switch (args->ptr[n].type)
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@@ -1187,6 +1225,42 @@ grub_xasprintf (const char *fmt, ...)
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   return ret;
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 }
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+grub_err_t
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+grub_printf_fmt_check (const char *fmt, const char *fmt_expected)
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+{
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+  struct printf_args args_expected, args_fmt;
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+  grub_err_t ret;
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+  grub_size_t n;
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+
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+  if (fmt == NULL || fmt_expected == NULL)
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+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "invalid format");
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+
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+  ret = parse_printf_arg_fmt (fmt_expected, &args_expected, 1, GRUB_SIZE_MAX);
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+  if (ret != GRUB_ERR_NONE)
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+    return ret;
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+
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+  /* Limit parsing to the number of expected arguments. */
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+  ret = parse_printf_arg_fmt (fmt, &args_fmt, 1, args_expected.count);
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+  if (ret != GRUB_ERR_NONE)
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+    {
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+      free_printf_args (&args_expected);
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+      return ret;
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+    }
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+
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+  for (n = 0; n < args_fmt.count; n++)
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+    if (args_fmt.ptr[n].type != args_expected.ptr[n].type)
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+     {
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+	ret = grub_error (GRUB_ERR_BAD_ARGUMENT, "arguments types do not match");
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+	break;
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+     }
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+
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+  free_printf_args (&args_expected);
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+  free_printf_args (&args_fmt);
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+
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+  return ret;
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+}
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+
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+
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 /* Abort GRUB. This function does not return.  */
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 static inline void __attribute__ ((noreturn))
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 grub_abort (void)
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diff --git a/include/grub/misc.h b/include/grub/misc.h
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index 6ca03c4d692..6be6a88f652 100644
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--- a/include/grub/misc.h
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+++ b/include/grub/misc.h
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@@ -488,6 +488,22 @@ grub_error_load (const struct grub_error_saved *save)
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   grub_errno = save->grub_errno;
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 }
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+/*
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+ * grub_printf_fmt_checks() a fmt string for printf() against an expected
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+ * format. It is intended for cases where the fmt string could come from
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+ * an outside source and cannot be trusted.
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+ *
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+ * While expected fmt accepts a printf() format string it should be kept
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+ * as simple as possible. The printf() format strings with positional
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+ * parameters are NOT accepted, neither for fmt nor for fmt_expected.
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+ *
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+ * The fmt is accepted if it has equal or less arguments than fmt_expected
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+ * and if the type of all arguments match.
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+ *
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+ * Returns GRUB_ERR_NONE if fmt is acceptable.
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+ */
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+grub_err_t EXPORT_FUNC (grub_printf_fmt_check) (const char *fmt, const char *fmt_expected);
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+
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 #if BOOT_TIME_STATS
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 struct grub_boot_time
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 {