Re: [PATCH] kdb: Get rid of custom debug heap allocator

[Sumit Garg <sumit.garg@linaro.org>]

On Thu, 25 Feb 2021 at 17:49, Daniel Thompson
<daniel.thompson@linaro.org> wrote:
>
> On Thu, Feb 25, 2021 at 04:52:58PM +0530, Sumit Garg wrote:
> > Currently the only user for debug heap is kdbnearsym() which can be
> > modified to rather ask the caller to supply a buffer for symbol name.
> > So do that and modify kdbnearsym() callers to pass a symbol name buffer
> > allocated from stack and hence remove custom debug heap allocator.
>
> Is it really a good idea to increase stack usage this much? I thought
> several architectures will take the debug exception on existing stacks
> (and that these can nest with other exceptions).
>
> The reason I'm concerned is that AFAICT the *purpose* of the current
> heap is to minimise stack usage... and that this has the effect of
> improving debugger robustness when we take exceptions on small shared
> stacks.
>
> The reason I called the heap redundant is that currently it also allows
> us to have nested calls to kdbnearsym() whilst not consuming stack. In
> this case, when I say nested I mean new calls to kdbnearsym() before the
> previous caller has consumed the output rather than truely recursive
> calls.
>
> This is why I think the heap is pointless. In "normal" usage I don't
> think there will never be a nested call to kdbnearsym() so I think a
> single static buffer will suffice.
>
> Technically speaking there is one way that kdbnearsym() can nest but I
> think it is OK for that to be considered out-of-scope.
>
> To explain...
>
> It can nest is if we recursively enter the debugger! Recursive entry
> should never happen, is pretty much untestable and, even if we tested
> it, it is not a bug for an architeture to choose not to support it.
> Nevertheless kgdb/kdb does include logic to handle this if an
> architecture does make it as far are executing the trap. Note that
> even if the architecture does somehow land in the debug trap there's
> a strong chance the system is is too broken to resume (since we just
> took an impossible trap). Therefore kdb will inhibit resume unless the
> operator admits what they are doing won't work before trying to do it.
>
> Therefore I think it is ok for namebuf to be statically allocated and
> the only thing we need do for stability is ensure that kdbnearsym()
> guarantees that namebuf[sizeof(namebuf)-1] == '\0' regardless of the
> symbol length. Thus if by some miracle the system can resume after the
> user has ignored the warning then kdb can't take a bad memory access
> when it tries to print an overwritten symbol name. They see a few
> garbage characters... but since they just told us to do something
> crazy they should be expecting that.
>

Thanks for the detailed explanation. I see the reasoning to not use
stack and it does sound reasonable to use statically allocated namebuf
with a stability guarantee.

>
> Daniel.
>
>
> PS The code to guarantee that if we read past the end of the string
>    we will still see a '\'0' before making an invalid memory access
>    should be well commented though... because its pretty nasty.
>

Sure, I will add a proper comment.

-Sumit

>
> >
> > This change has been tested using kgdbtest on arm64 which doesn't show
> > any regressions.
> >
> > Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
> > Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> > ---
> >  kernel/debug/kdb/kdb_debugger.c |   1 -
> >  kernel/debug/kdb/kdb_main.c     |   6 +-
> >  kernel/debug/kdb/kdb_private.h  |   7 +-
> >  kernel/debug/kdb/kdb_support.c  | 294 +-------------------------------
> >  4 files changed, 11 insertions(+), 297 deletions(-)
> >
> > diff --git a/kernel/debug/kdb/kdb_debugger.c b/kernel/debug/kdb/kdb_debugger.c
> > index 0220afda3200..e91fc3e4edd5 100644
> > --- a/kernel/debug/kdb/kdb_debugger.c
> > +++ b/kernel/debug/kdb/kdb_debugger.c
> > @@ -140,7 +140,6 @@ int kdb_stub(struct kgdb_state *ks)
> >        */
> >       kdb_common_deinit_state();
> >       KDB_STATE_CLEAR(PAGER);
> > -     kdbnearsym_cleanup();
> >       if (error == KDB_CMD_KGDB) {
> >               if (KDB_STATE(DOING_KGDB))
> >                       KDB_STATE_CLEAR(DOING_KGDB);
> > diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
> > index 9d69169582c6..ca525a3e0032 100644
> > --- a/kernel/debug/kdb/kdb_main.c
> > +++ b/kernel/debug/kdb/kdb_main.c
> > @@ -526,6 +526,7 @@ int kdbgetaddrarg(int argc, const char **argv, int *nextarg,
> >       char symbol = '\0';
> >       char *cp;
> >       kdb_symtab_t symtab;
> > +     char namebuf[KSYM_NAME_LEN];
> >
> >       /*
> >        * If the enable flags prohibit both arbitrary memory access
> > @@ -585,7 +586,7 @@ int kdbgetaddrarg(int argc, const char **argv, int *nextarg,
> >       }
> >
> >       if (!found)
> > -             found = kdbnearsym(addr, &symtab);
> > +             found = kdbnearsym(addr, &symtab, namebuf);
> >
> >       (*nextarg)++;
> >
> > @@ -1503,6 +1504,7 @@ static void kdb_md_line(const char *fmtstr, unsigned long addr,
> >       int i;
> >       int j;
> >       unsigned long word;
> > +     char namebuf[KSYM_NAME_LEN];
> >
> >       memset(cbuf, '\0', sizeof(cbuf));
> >       if (phys)
> > @@ -1518,7 +1520,7 @@ static void kdb_md_line(const char *fmtstr, unsigned long addr,
> >                       break;
> >               kdb_printf(fmtstr, word);
> >               if (symbolic)
> > -                     kdbnearsym(word, &symtab);
> > +                     kdbnearsym(word, &symtab, namebuf);
> >               else
> >                       memset(&symtab, 0, sizeof(symtab));
> >               if (symtab.sym_name) {
> > diff --git a/kernel/debug/kdb/kdb_private.h b/kernel/debug/kdb/kdb_private.h
> > index b857a84de3b5..1707eeebc59a 100644
> > --- a/kernel/debug/kdb/kdb_private.h
> > +++ b/kernel/debug/kdb/kdb_private.h
> > @@ -108,8 +108,7 @@ extern char *kdbgetenv(const char *);
> >  extern int kdbgetaddrarg(int, const char **, int*, unsigned long *,
> >                        long *, char **);
> >  extern int kdbgetsymval(const char *, kdb_symtab_t *);
> > -extern int kdbnearsym(unsigned long, kdb_symtab_t *);
> > -extern void kdbnearsym_cleanup(void);
> > +extern int kdbnearsym(unsigned long addr, kdb_symtab_t *symtab, char *namebuf);
> >  extern char *kdb_strdup(const char *str, gfp_t type);
> >  extern void kdb_symbol_print(unsigned long, const kdb_symtab_t *, unsigned int);
> >
> > @@ -233,10 +232,6 @@ extern struct task_struct *kdb_curr_task(int);
> >
> >  #define GFP_KDB (in_dbg_master() ? GFP_ATOMIC : GFP_KERNEL)
> >
> > -extern void *debug_kmalloc(size_t size, gfp_t flags);
> > -extern void debug_kfree(void *);
> > -extern void debug_kusage(void);
> > -
> >  extern struct task_struct *kdb_current_task;
> >  extern struct pt_regs *kdb_current_regs;
> >
> > diff --git a/kernel/debug/kdb/kdb_support.c b/kernel/debug/kdb/kdb_support.c
> > index b59aad1f0b55..319b36ceedf9 100644
> > --- a/kernel/debug/kdb/kdb_support.c
> > +++ b/kernel/debug/kdb/kdb_support.c
> > @@ -57,8 +57,6 @@ int kdbgetsymval(const char *symname, kdb_symtab_t *symtab)
> >  }
> >  EXPORT_SYMBOL(kdbgetsymval);
> >
> > -static char *kdb_name_table[100];    /* arbitrary size */
> > -
> >  /*
> >   * kdbnearsym -      Return the name of the symbol with the nearest address
> >   *   less than 'addr'.
> > @@ -79,13 +77,11 @@ static char *kdb_name_table[100]; /* arbitrary size */
> >   *   hold active strings, no kdb caller of kdbnearsym makes more
> >   *   than ~20 later calls before using a saved value.
> >   */
> > -int kdbnearsym(unsigned long addr, kdb_symtab_t *symtab)
> > +int kdbnearsym(unsigned long addr, kdb_symtab_t *symtab, char *namebuf)
> >  {
> >       int ret = 0;
> >       unsigned long symbolsize = 0;
> >       unsigned long offset = 0;
> > -#define knt1_size 128                /* must be >= kallsyms table size */
> > -     char *knt1 = NULL;
> >
> >       if (KDB_DEBUG(AR))
> >               kdb_printf("kdbnearsym: addr=0x%lx, symtab=%px\n", addr, symtab);
> > @@ -93,14 +89,9 @@ int kdbnearsym(unsigned long addr, kdb_symtab_t *symtab)
> >
> >       if (addr < 4096)
> >               goto out;
> > -     knt1 = debug_kmalloc(knt1_size, GFP_ATOMIC);
> > -     if (!knt1) {
> > -             kdb_printf("kdbnearsym: addr=0x%lx cannot kmalloc knt1\n",
> > -                        addr);
> > -             goto out;
> > -     }
> > +
> >       symtab->sym_name = kallsyms_lookup(addr, &symbolsize , &offset,
> > -                             (char **)(&symtab->mod_name), knt1);
> > +                             (char **)(&symtab->mod_name), namebuf);
> >       if (offset > 8*1024*1024) {
> >               symtab->sym_name = NULL;
> >               addr = offset = symbolsize = 0;
> > @@ -109,42 +100,6 @@ int kdbnearsym(unsigned long addr, kdb_symtab_t *symtab)
> >       symtab->sym_end = symtab->sym_start + symbolsize;
> >       ret = symtab->sym_name != NULL && *(symtab->sym_name) != '\0';
> >
> > -     if (ret) {
> > -             int i;
> > -             /* Another 2.6 kallsyms "feature".  Sometimes the sym_name is
> > -              * set but the buffer passed into kallsyms_lookup is not used,
> > -              * so it contains garbage.  The caller has to work out which
> > -              * buffer needs to be saved.
> > -              *
> > -              * What was Rusty smoking when he wrote that code?
> > -              */
> > -             if (symtab->sym_name != knt1) {
> > -                     strncpy(knt1, symtab->sym_name, knt1_size);
> > -                     knt1[knt1_size-1] = '\0';
> > -             }
> > -             for (i = 0; i < ARRAY_SIZE(kdb_name_table); ++i) {
> > -                     if (kdb_name_table[i] &&
> > -                         strcmp(kdb_name_table[i], knt1) == 0)
> > -                             break;
> > -             }
> > -             if (i >= ARRAY_SIZE(kdb_name_table)) {
> > -                     debug_kfree(kdb_name_table[0]);
> > -                     memmove(kdb_name_table, kdb_name_table+1,
> > -                            sizeof(kdb_name_table[0]) *
> > -                            (ARRAY_SIZE(kdb_name_table)-1));
> > -             } else {
> > -                     debug_kfree(knt1);
> > -                     knt1 = kdb_name_table[i];
> > -                     memmove(kdb_name_table+i, kdb_name_table+i+1,
> > -                            sizeof(kdb_name_table[0]) *
> > -                            (ARRAY_SIZE(kdb_name_table)-i-1));
> > -             }
> > -             i = ARRAY_SIZE(kdb_name_table) - 1;
> > -             kdb_name_table[i] = knt1;
> > -             symtab->sym_name = kdb_name_table[i];
> > -             knt1 = NULL;
> > -     }
> > -
> >       if (symtab->mod_name == NULL)
> >               symtab->mod_name = "kernel";
> >       if (KDB_DEBUG(AR))
> > @@ -152,23 +107,10 @@ int kdbnearsym(unsigned long addr, kdb_symtab_t *symtab)
> >                  "symtab->mod_name=%px, symtab->sym_name=%px (%s)\n", ret,
> >                  symtab->sym_start, symtab->mod_name, symtab->sym_name,
> >                  symtab->sym_name);
> > -
> >  out:
> > -     debug_kfree(knt1);
> >       return ret;
> >  }
> >
> > -void kdbnearsym_cleanup(void)
> > -{
> > -     int i;
> > -     for (i = 0; i < ARRAY_SIZE(kdb_name_table); ++i) {
> > -             if (kdb_name_table[i]) {
> > -                     debug_kfree(kdb_name_table[i]);
> > -                     kdb_name_table[i] = NULL;
> > -             }
> > -     }
> > -}
> > -
> >  static char ks_namebuf[KSYM_NAME_LEN+1], ks_namebuf_prev[KSYM_NAME_LEN+1];
> >
> >  /*
> > @@ -259,11 +201,13 @@ void kdb_symbol_print(unsigned long addr, const kdb_symtab_t *symtab_p,
> >                     unsigned int punc)
> >  {
> >       kdb_symtab_t symtab, *symtab_p2;
> > +     char namebuf[KSYM_NAME_LEN];
> > +
> >       if (symtab_p) {
> >               symtab_p2 = (kdb_symtab_t *)symtab_p;
> >       } else {
> >               symtab_p2 = &symtab;
> > -             kdbnearsym(addr, symtab_p2);
> > +             kdbnearsym(addr, symtab_p2, namebuf);
> >       }
> >       if (!(symtab_p2->sym_name || (punc & KDB_SP_VALUE)))
> >               return;
> > @@ -665,232 +609,6 @@ unsigned long kdb_task_state(const struct task_struct *p, unsigned long mask)
> >       return (mask & kdb_task_state_string(state)) != 0;
> >  }
> >
> > -/* Last ditch allocator for debugging, so we can still debug even when
> > - * the GFP_ATOMIC pool has been exhausted.  The algorithms are tuned
> > - * for space usage, not for speed.  One smallish memory pool, the free
> > - * chain is always in ascending address order to allow coalescing,
> > - * allocations are done in brute force best fit.
> > - */
> > -
> > -struct debug_alloc_header {
> > -     u32 next;       /* offset of next header from start of pool */
> > -     u32 size;
> > -     void *caller;
> > -};
> > -
> > -/* The memory returned by this allocator must be aligned, which means
> > - * so must the header size.  Do not assume that sizeof(struct
> > - * debug_alloc_header) is a multiple of the alignment, explicitly
> > - * calculate the overhead of this header, including the alignment.
> > - * The rest of this code must not use sizeof() on any header or
> > - * pointer to a header.
> > - */
> > -#define dah_align 8
> > -#define dah_overhead ALIGN(sizeof(struct debug_alloc_header), dah_align)
> > -
> > -static u64 debug_alloc_pool_aligned[256*1024/dah_align];     /* 256K pool */
> > -static char *debug_alloc_pool = (char *)debug_alloc_pool_aligned;
> > -static u32 dah_first, dah_first_call = 1, dah_used, dah_used_max;
> > -
> > -/* Locking is awkward.  The debug code is called from all contexts,
> > - * including non maskable interrupts.  A normal spinlock is not safe
> > - * in NMI context.  Try to get the debug allocator lock, if it cannot
> > - * be obtained after a second then give up.  If the lock could not be
> > - * previously obtained on this cpu then only try once.
> > - *
> > - * sparse has no annotation for "this function _sometimes_ acquires a
> > - * lock", so fudge the acquire/release notation.
> > - */
> > -static DEFINE_SPINLOCK(dap_lock);
> > -static int get_dap_lock(void)
> > -     __acquires(dap_lock)
> > -{
> > -     static int dap_locked = -1;
> > -     int count;
> > -     if (dap_locked == smp_processor_id())
> > -             count = 1;
> > -     else
> > -             count = 1000;
> > -     while (1) {
> > -             if (spin_trylock(&dap_lock)) {
> > -                     dap_locked = -1;
> > -                     return 1;
> > -             }
> > -             if (!count--)
> > -                     break;
> > -             udelay(1000);
> > -     }
> > -     dap_locked = smp_processor_id();
> > -     __acquire(dap_lock);
> > -     return 0;
> > -}
> > -
> > -void *debug_kmalloc(size_t size, gfp_t flags)
> > -{
> > -     unsigned int rem, h_offset;
> > -     struct debug_alloc_header *best, *bestprev, *prev, *h;
> > -     void *p = NULL;
> > -     if (!get_dap_lock()) {
> > -             __release(dap_lock);    /* we never actually got it */
> > -             return NULL;
> > -     }
> > -     h = (struct debug_alloc_header *)(debug_alloc_pool + dah_first);
> > -     if (dah_first_call) {
> > -             h->size = sizeof(debug_alloc_pool_aligned) - dah_overhead;
> > -             dah_first_call = 0;
> > -     }
> > -     size = ALIGN(size, dah_align);
> > -     prev = best = bestprev = NULL;
> > -     while (1) {
> > -             if (h->size >= size && (!best || h->size < best->size)) {
> > -                     best = h;
> > -                     bestprev = prev;
> > -                     if (h->size == size)
> > -                             break;
> > -             }
> > -             if (!h->next)
> > -                     break;
> > -             prev = h;
> > -             h = (struct debug_alloc_header *)(debug_alloc_pool + h->next);
> > -     }
> > -     if (!best)
> > -             goto out;
> > -     rem = best->size - size;
> > -     /* The pool must always contain at least one header */
> > -     if (best->next == 0 && bestprev == NULL && rem < dah_overhead)
> > -             goto out;
> > -     if (rem >= dah_overhead) {
> > -             best->size = size;
> > -             h_offset = ((char *)best - debug_alloc_pool) +
> > -                        dah_overhead + best->size;
> > -             h = (struct debug_alloc_header *)(debug_alloc_pool + h_offset);
> > -             h->size = rem - dah_overhead;
> > -             h->next = best->next;
> > -     } else
> > -             h_offset = best->next;
> > -     best->caller = __builtin_return_address(0);
> > -     dah_used += best->size;
> > -     dah_used_max = max(dah_used, dah_used_max);
> > -     if (bestprev)
> > -             bestprev->next = h_offset;
> > -     else
> > -             dah_first = h_offset;
> > -     p = (char *)best + dah_overhead;
> > -     memset(p, POISON_INUSE, best->size - 1);
> > -     *((char *)p + best->size - 1) = POISON_END;
> > -out:
> > -     spin_unlock(&dap_lock);
> > -     return p;
> > -}
> > -
> > -void debug_kfree(void *p)
> > -{
> > -     struct debug_alloc_header *h;
> > -     unsigned int h_offset;
> > -     if (!p)
> > -             return;
> > -     if ((char *)p < debug_alloc_pool ||
> > -         (char *)p >= debug_alloc_pool + sizeof(debug_alloc_pool_aligned)) {
> > -             kfree(p);
> > -             return;
> > -     }
> > -     if (!get_dap_lock()) {
> > -             __release(dap_lock);    /* we never actually got it */
> > -             return;         /* memory leak, cannot be helped */
> > -     }
> > -     h = (struct debug_alloc_header *)((char *)p - dah_overhead);
> > -     memset(p, POISON_FREE, h->size - 1);
> > -     *((char *)p + h->size - 1) = POISON_END;
> > -     h->caller = NULL;
> > -     dah_used -= h->size;
> > -     h_offset = (char *)h - debug_alloc_pool;
> > -     if (h_offset < dah_first) {
> > -             h->next = dah_first;
> > -             dah_first = h_offset;
> > -     } else {
> > -             struct debug_alloc_header *prev;
> > -             unsigned int prev_offset;
> > -             prev = (struct debug_alloc_header *)(debug_alloc_pool +
> > -                                                  dah_first);
> > -             while (1) {
> > -                     if (!prev->next || prev->next > h_offset)
> > -                             break;
> > -                     prev = (struct debug_alloc_header *)
> > -                             (debug_alloc_pool + prev->next);
> > -             }
> > -             prev_offset = (char *)prev - debug_alloc_pool;
> > -             if (prev_offset + dah_overhead + prev->size == h_offset) {
> > -                     prev->size += dah_overhead + h->size;
> > -                     memset(h, POISON_FREE, dah_overhead - 1);
> > -                     *((char *)h + dah_overhead - 1) = POISON_END;
> > -                     h = prev;
> > -                     h_offset = prev_offset;
> > -             } else {
> > -                     h->next = prev->next;
> > -                     prev->next = h_offset;
> > -             }
> > -     }
> > -     if (h_offset + dah_overhead + h->size == h->next) {
> > -             struct debug_alloc_header *next;
> > -             next = (struct debug_alloc_header *)
> > -                     (debug_alloc_pool + h->next);
> > -             h->size += dah_overhead + next->size;
> > -             h->next = next->next;
> > -             memset(next, POISON_FREE, dah_overhead - 1);
> > -             *((char *)next + dah_overhead - 1) = POISON_END;
> > -     }
> > -     spin_unlock(&dap_lock);
> > -}
> > -
> > -void debug_kusage(void)
> > -{
> > -     struct debug_alloc_header *h_free, *h_used;
> > -#ifdef       CONFIG_IA64
> > -     /* FIXME: using dah for ia64 unwind always results in a memory leak.
> > -      * Fix that memory leak first, then set debug_kusage_one_time = 1 for
> > -      * all architectures.
> > -      */
> > -     static int debug_kusage_one_time;
> > -#else
> > -     static int debug_kusage_one_time = 1;
> > -#endif
> > -     if (!get_dap_lock()) {
> > -             __release(dap_lock);    /* we never actually got it */
> > -             return;
> > -     }
> > -     h_free = (struct debug_alloc_header *)(debug_alloc_pool + dah_first);
> > -     if (dah_first == 0 &&
> > -         (h_free->size == sizeof(debug_alloc_pool_aligned) - dah_overhead ||
> > -          dah_first_call))
> > -             goto out;
> > -     if (!debug_kusage_one_time)
> > -             goto out;
> > -     debug_kusage_one_time = 0;
> > -     kdb_printf("%s: debug_kmalloc memory leak dah_first %d\n",
> > -                __func__, dah_first);
> > -     if (dah_first) {
> > -             h_used = (struct debug_alloc_header *)debug_alloc_pool;
> > -             kdb_printf("%s: h_used %px size %d\n", __func__, h_used,
> > -                        h_used->size);
> > -     }
> > -     do {
> > -             h_used = (struct debug_alloc_header *)
> > -                       ((char *)h_free + dah_overhead + h_free->size);
> > -             kdb_printf("%s: h_used %px size %d caller %px\n",
> > -                        __func__, h_used, h_used->size, h_used->caller);
> > -             h_free = (struct debug_alloc_header *)
> > -                       (debug_alloc_pool + h_free->next);
> > -     } while (h_free->next);
> > -     h_used = (struct debug_alloc_header *)
> > -               ((char *)h_free + dah_overhead + h_free->size);
> > -     if ((char *)h_used - debug_alloc_pool !=
> > -         sizeof(debug_alloc_pool_aligned))
> > -             kdb_printf("%s: h_used %px size %d caller %px\n",
> > -                        __func__, h_used, h_used->size, h_used->caller);
> > -out:
> > -     spin_unlock(&dap_lock);
> > -}
> > -
> >  /* Maintain a small stack of kdb_flags to allow recursion without disturbing
> >   * the global kdb state.
> >   */
> > --
> > 2.25.1
> >